Basics of immunology

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BASICS OF IMMUNOLOGY
Abdul Ahad
Muhammad Anees
Zahida Fatima
2011

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ABOUT THE AUTHOR
Abdul Ahad is Professor in the department of Microbiology, Chittagong Veterinary and
Animal Sciences University, Chittagong, Bangladesh. He teaches virology, immunology and
serology, and food microbiology. So far he published 16 articles in different national and
international journals. He did his DVM and MS in the year 1993 and 1995 respectively from
Bangladesh Agriculture University, Mymensingh. He completed his MVSc from the Royal
Veterinary and Agriculture University, Copenhagen, Denmark in the year 20002.
Muhammad Anees passed DVM and M.Sc. (Hons.) in Veterinary Microbiology in the year 1990
and 1993 respectively from University of Agriculture Faisalabad, Pakistan. He joined Livestock
and Dairy Development, Department as Veterinary Officer (Health) in the year 1992 and serving
as a senior veterinary officer. He is well reputed pet animal practitioner.
Zahida Fatima is an Epidemiologist in Animal health program, National Agricultural Research
Center, Islamabad, Pakistan. Her research interests are in the emerging and re-emerging
infectious diseases of animal and humans with particular emphasis on zoonotic disease. She
earned her DVM and MSc (Hons.) degrees in the year 1999 and 2002 from University of
Veterinary and Animal Sciences, Lahore, Pakistan. She was trained in the field of epidemiology,
public health, zoononsis and biostatistics in several courses in different countries like University
of Florida, USA, Jordan university of Science and technology Jordan, USDAAPHIS, Fortcollins,
Colorado, USA. She was also Master Trainer for Participatory Disease Epidemiology and had
contributed significantly in the Rinderpest Eradication Program in Pakistan being the member of
team.

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PREFACE TO FIRST EDITION
Preface
This first edition of “Basics of Immunology” is a glimpse of immunology. Initially the author
prepared a handout on immunology for his preparation for his PhD comprehensive written
examination. After the examination he thought to give it in a shape of book with a view that this
will be guide for future PhD students who will prepare for their written and oral comprehensive
examination. Hopefully this book will be a guide for teachers and students to understand the
basic concepts of immunology. Future coming students and my class mates were the major
inspiration to write this book. The authors are very grateful to Dr. Tanveer Hussain, Lecturer,
Institute of Biochemistry and Biotechnology, University of Veterinary and Animal Science who
gives the idea to publish the book and helps also in computer composing of this monograph.
ABDUL AHAD
MUHAMMAD ANEES
ZAHIDA FATIMA

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DEDICATED TO
MY BELOVED PARENTS WHO ALWAYS ENCOURAGE ME FOR MY HIGHER STUDIES
SINCE MY CHILDHOOD

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TABLE OF CONTENTS
CHAPTER 1 .......................................................................................................... 10
GENERAL PROPERTIES OF IMMUNE RESPONSE.............................................................. 10
CHAPTER 2 .......................................................................................................... 15
OVERVIEW OF IMMUNE RESPONSE............................................................................... 15
CHAPTER 3 .......................................................................................................... 20
ANTIGEN.................................................................................................................... 20
CHAPTER 4 .......................................................................................................... 23
ANTIBODIES ............................................................................................................... 23
CHAPTER 5 .......................................................................................................... 30
CYTOKINES................................................................................................................ 30
CHAPTER 6 .......................................................................................................... 39
LYMPHOID ORGAN ..................................................................................................... 39
CHAPTER 7 .......................................................................................................... 47
PHAGOCYTOSIS.......................................................................................................... 47
CHAPTER 8 .......................................................................................................... 52
COMPLEMENT.......................................................................................................... 52
CHAPTER 9 .......................................................................................................... 57
HYBRIDOMA & MONOCLONAL ANTIBODY..................................................................... 57
CHAPTER 10 ........................................................................................................ 61
MAJOR HISTOCOMPATIBILITY COMPLEX ..................................................................... 61
CHAPTER 11 ........................................................................................................ 63
ANTIGEN PROCESSING AND PRESENTATION.................................................................. 63
CHAPTER 12 ........................................................................................................ 69
ANTIGEN RECEPTORS AND ACCESSORY MOLECULES OF T LYMPHOCYTES ...................... 69
CHAPTER 13 ........................................................................................................ 78
B CELL ACTIVATION AND ANTIBODY PRODUCTION........................................................ 78
CHAPTER 14 ........................................................................................................ 87
IMMUNOLOGICAL TOLERANCE.................................................................................... 87
CHAPTER 15 ........................................................................................................ 96
AUTOIMMUNITY: GENERAL PRINCIPLES....................................................................... 96
CHAPTER 16 ........................................................................................................ 99
ENDOCRINE (ORGAN SPECIFIC AUTOIMMUNE DISEASES)................................................ 99
CHAPTER 17 ...................................................................................................... 102
SYSTEMIC IMMUNOLOGICAL DISEASES ...................................................................... 102
CHAPTER 18 ...................................................................................................... 104
IMMUNE DEFICIENCY DISEASE................................................................................... 104

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CHAPTER 19 ...................................................................................................... 106
SECONDARY IMMUNE DEFICIENCIES .......................................................................... 106
CHAPTER 20 ...................................................................................................... 107
TRANSPLANTATIONAL IMMUNOLOGY ........................................................................ 107
CHAPTER 21 ...................................................................................................... 110
TUMOR IMMUNOLOGY.............................................................................................. 110
CHAPTER 22 ...................................................................................................... 114
INDUCTION OF MUCOSAL IMMUNITY.......................................................................... 114
CHAPTER 23 ...................................................................................................... 117
HYPERSENSITIVITY .................................................................................................. 117

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LIST OF TABLES
1 Features of Innate and Adaptive Immunity 11
2 Difference between different classes of immunoglobulin 25
3 Different classes of immunoglobulin 29
4 Comparative features of the cytokines of innate and adaptive immunity 34
5 Features of Primary and Secondary Antibody Responses 95

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LIST OF FIGURES
1 Innate and adaptive immunity 12
2 Types of adaptive immunity 13
3 Overview of immune responses in vivo 16
4 Structure of an antibody molecule 27
5 Proteolytic fragments of an IgG molecule 28
6 Functions of cytokines in host defense 31
7 Functions of cytokines in host defense 32
8 Properties of cytokines 34
9 Biologic actions of TNF 36
10 Biologic function of IL 37
11 Histology of bursa of Fabricius 39
12 Thymus 41
13 Schematic diagram of a lymph node illustrating the T cell rich and B cell rich zones
and the routes of entry of lymphocytes and antigen (shown captured by dendritic cell)
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14 Schematic diagram illustrates the path by which naive T and B lymphocytes migrate
to different areas of lymph node
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15 Spleen 46
16 Migration of naive and effector T lymphocyte 48
17 Phases of phagocytosis 49
18 Outcomes of complement activation 53
19 Classical pathway of complement activation 54
20 Alternative pathway of complement activation 55
21 Lectin pathway of complement activation 56
22 A schematic diagram showing the method of production of monoclonal antibodies 59
23 The pathways of purine synthesis and the mechanism of action of HAT medium 60
24 Structure of a class I MHC molecule 61
25 Structure of a class II MHC molecule 62
26 Cross-presentation of antigens to CD8+ T cells 65
27 The class II MHC pathway of antigen processing 66
28 The class I MHC pathway of antigen presentation 68

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29 T cell receptors and accessory molecules 70
30 Structure of T cell Receptor 71
31 The interaction of a CD4+ helper T cell with an APC (A) involves multiple T cell
membrane proteins that recognize different ligands on the APC or target cell
72
32 The interaction a CD8+ CTL with a target cell (B), involves multiple T cell membrane
proteins that recognize different ligands on the APC or target cell
73
33 Functions of costimulators in T cell activation 74
34 Role of B7 and CD28 in T cell activation 74
38 Role of CD40 in T cell activation 76
39 Formation of the immunological synapse 77
40 Phases of the humoral immune response 78
41 Kinetics of primary and secondary humoral immune responses 79
42 Signal transduction 80
43 Role of complement in B cell activation 81
44 Functional responses induced by antigen-mediated cross-linking of the BCR complex 82
45 Early and late events in humoral immune response to T cell dependent protein
antigens
84
46 Germinal centre reactions in T – Cells dependent response 85
47 Regulation of B cell activation by Ig Fc receptors 86
48 Fates of lymphocytes after encounter with antigens 87
49 Central T cell tolerance 88
50 T cell anergy induced by a self antigen in transgenic mice 89
51 T cell anergy 90
52 Activation-induced death of T lymphocytes 91
53 Biochemical mechanisms of apoptosis 92
54 Central tolerance in B lymphocytes in a transgenic mouse model 93
55 Peripheral B cell tolerance in a transgenic mouse model 94

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CHAPTER 1
GENERAL PROPERTIES OF IMMUNE RESPONSE
Immunity: The term immunity is coined from Latin word Immunitus means protection.
Molecules and cells constitute the Immune system and their collective and harmonized
response to the introduced foreign substance is called Immune response.
Types of immunity: Innate (early) & adaptive (late)
Components of innate:
1. Epithelial surface. Intact epithelial surface form physical barriers between microbes in the
external environment & host tissue. Epithelia, as well as leukocytes, produce peptides that have
antimicrobial properties. e.g. Defensins, cathelicidins
2. Phagocytic cells (Neutrophil, macrophage) and natural killer (NK) cells.
3. Blood proteins including members of the complement system.
4. Cytokines that regulate and synchronize many activities of the cells on innate immunity.
Mechanism: Innate immunity can detect common structure of related microbes & may not
capable to discriminate fine differences among foreign substances.
Adaptive: This form of immunity evolves as a response to infection and acclimitize to the
infection, it is called adaptive immunity. It is stimulated by exposure to antigen & increase in
magnitude by successive exposure. Defining characteristics of adaptive immunity are exquisite
specifity for distinct molecules & ability to remember (specific immunity).
Components of adaptive immunity: Lymphocytes & their secreted products (cytokines &
antibodies).
Antigen: foreign substances that can provoke / induce specific immune response.

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Pathogenic microbes resist innate immunity hence they are eliminated by specific immunity
Table 1: Features of Innate and Adaptive Immunity
Types of adaptive immunity
1. Humoral
2. Cell mediated
Humoral immunity is regulated by antibodies, which are produced by B lymphocytes.
Antibodies identify microbial antigens, negate the infectivity of the microbes. It eliminates
microbes by various effectors mechanisms. Humoral immunity is the chief defense mechanism
against extracellular microbes and their toxins owing secreted antibodies can bind to these
microbes and toxins & help in their elimination.
Cell mediated immunity is regulated by T lymphocytes. Intracellular microbes (virus & some
bacteria) exist and multiply inside phagocytes and other host cells, where they are not
permissible to diffuse antibodies. Defense against such infection is a function of cell mediated
immunity, which assists the destruction of microbes remaining in phagocyte or the killing of
infected cells to get rid reservoirs of infection.

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Protective immunity may be induced by host response to microbe or by transfer of antibody or
lymphocytes
Active immunity is induced by exposure to foreign antigen.
Passive immunity: Adoptive transfer of serum or lymphocytes
Figure 1- Innate and adaptive immunity
Initial
defense
Develop
later

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Fig 2: Types of adaptive immunity
Cellular components of adaptive immune response (lymphocyte, antigen presenting cell, effector
cells)
Classes of lymphocytes
Divided on the basis of surface CD (cluster of differentiation) molecules
B lymphocytes: They are the only cells capable of producing antibodies. They recognize
extracellular antigens & differentiate into antibody secreting plasma cells. Neutralization of
microbe, phagocytosis, complement activation.
Helper T cells: In response to antigenic stimulation, helper T cells secrete proteins called
cytokines, whose functions are to stimulate the proliferation & differentiation of the T cells
themselves, activate other cells, including B cells, macrophage and other leucocytes.

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T cytotoxic (CTLs): CTLs kill cells that produce foreign antigens, such as cells infected by
viruses & other intracellular microbes.
Regulatory T cells: Inhibit immune response
NK cells (Natural Killer): NK cells are involved in innate immunity against viruses & other
intracellular microbes.

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CHAPTER 2
OVERVIEW OF IMMUNE RESPONSE
The Early Innate Immune Response to Microbes
The innate immune system blocks the entry of microbes & eliminates or limits the growth of
many microbes that are able to colonize tissues. The main sites of interaction between
individuals & their environment – the skin, & gastro intestinal & respiratory tracts are lined by
continuous epithelia, which serve as barriers to prevent the entry of microbes from the external
environment. If microbes successfully breach the epithelial barriers, they encounter macrophage
in sub epithelial tissue. Macrophage express on their surface receptors that bind & ingest
microbes, & other receptors that recognize different microbial molecules & activate the cells.
Activated macrophages perform several functions that collectively serve to eliminate ingested
microbes. These cells produce reactive oxygen species & lysosomal enzymes which destroy
microbes that have been ingested. Macrophage secretes cytokines that promote the recruitment
of other leukocytes, such as neutrophils, from blood vessel to the site of infection. The innate
immune response to some infectious pathogens, particularly viruses, consists of the production of
antiviral cytokines called interferon & activation of NK cells, which kill virus infected cells.
The Adaptive Immune Response
The adaptive immune system uses three main strategies to combat most microbes
Secreted antibodies bind to extracellular microbes, block their ability to infect host cells, &
promote their ingestion & subsequent destruction by phagocytes.
Phagocytes ingest microbes & kill them, and helper T cells enhance the microbial abilities of the
phagocytes.
Cytotoxic T lymphocytes destroy cells infected by microbes that are inaccessible to antibodies.

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Figure 3: Overview of immune responses in vivo.

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Explanation of overview
Capture and display of antigen: The number of naïve lymphocytes specific for any antigen is
very small (on the order of 1 in 105
or 106
lymphocytes).
Dendritic cells are the APCs that display microbial peptides to naïve CD4+
and CD8+
T
lymphcoytes and initiate adaptive immune responses to protein antigens. Intact microbe or
microbial antigens that enter lymph nodes and spleen are recognized in unprocessed (native)
form by specific B lymphocytes.
Antigen recognition
A. Clonal Selection Hypothesis
Lymphocytes specific for a large number of antigens exist prior to exposure to antigen. When an
antigen enters, it selects specific cells & activate them. This fundamental concept is called the
Clonal Selection Hypothesis. It is estimated that there are >106
different specificities in T and B
lymphocytes so that at least this many antigenic determinants can be recognized by the adaptive
immune system.
Activation of naïve T lymphocytes requires recognition of peptide-MHC complexes presented on
dendritic cells. In order to respond, the T cells need to recognize not only antigens but also other
molecules, called co-stimulators that are induced on APCs by microbes.
B cells use their antigen receptor to recognize antigens of many different chemical types.
Cell mediated immunity : Activation of T - Lymphocytes & Elimination of Intracellular
Microbes
Activated CD4+
helper T lymphocytes proliferate and differentiate into effector cells whose
functions are mediated largely by secreted cytokines. One of the earliest response of CD4+
helper
T cells is secretion of the cytokine interleukin-2 (IL-2). IL-2 is a growth factor that acts on

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antigen activated lymphocytes and stimulates their proliferation (Clonal expansion). Some of the
progeny differentiate into effector cells that can secrete different sets of cytokines, and thus
different functions. These effector cells leave the lymphoid organs where they were generated
and migrate to site of infection and accompanying inflammation. When these differentiated
effectors again encounter cell associate microbes, they are activated to perform the functions that
are responsible for elimination of microbes. Some effector T cells of the CD4+
helper cell lineage
secrete the cytokine IFNγ which is a potent macrophage activator and induces production of
microbicidal substances in macrophages.
Other CD4+ effector T cell secrete cytokines that stimulate the production of a special class of
antibody called IgE activate eosinophils, which are able to kill parasites.
Activated CD8+
lymphocytes proliferate & differential into CTLs (cytotoxic T lymphocyte) that
kill cells harboring microbes in the cytoplasm. These microbes may be viruses that infect many
cell types or bacteria that are ingested by mφ.

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Humoral immunity
Upon activation B cells proliferate & differentiate into cells that secrete different class of
antibodies with different function. Many polysaccharide and lipid antigens have multiple
identical antigenic determinants that are able to engage many antigen receptor molecules on each
B cell & initiate the process of B cell activation. The response of B cells to protein antigens
require activating signals from CD4+ T cells. B cells ingest protein antigens, degrade them, and
display peptides bound to MHC molecules for recognition by helper T cells, which then activate
the B cells.
Polysaccharides and lipids stimulate secretion mainly class IgM. Protein antigen by virtue of
helper T cell actions, induce the production of antibodies of different classes (IgG, IgA, IgE).
IgG is actively transported across the placenta, & protects the newborn until the immune system
becomes mature. Most antibodies have half lives of about 3 weeks.
(Immunological memory)
The initial activation of lymphocytes generates long live memory cells, which may survive for
years after the infection.

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CHAPTER 3
ANTIGEN
Foreign substance that induce specific immune response or are the targets of such responses.
Microbial antigens
Bacterial antigen
Cell wall (O), capsule (K), pilli (F), flagella (H)
Viral antigen
Endogenous antigen, surface glycoprotein, capsid, lipoprotein
Non microbial antigens:
Food, inhaled dust cell surface antigen (blood group antigen), glycoprotein auto antigen
(Harmless)
What makes a good antigen?
Antigenicity depends upon
1. Foreignness
2. Complexity e.g. bacterial flagella is not a good antigen because it is polymer of alanine
3. Size
4. Dose
5. Chemical stability
6. Route of administration
7. Host Genetics
Ranking
1. Protein
2. Carbohydrate
3. Lipid
4. Nucleic acid poor until complexed with proteins

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Properties of antigen
Foreignness: Phylogenetic distance between 2 species
Molecular size: Less than 1000 dalton poor
Chemical composition & heterogenicity: simple polymers, less immunogenic
Degradability: Inert substance do not induce immune response
Genotype of recipient: High responders versus low responders based on MHC gene producers /
molecule
Immunogen dose & route
Low or high dose → tolerance, gut versus parental
Mode of action of adjuvant
Prolonged retention of an antigen
Enhanced co-stimulatory signal
Stimulating the influx of populations of macrophages and / or lymphocytes to the site of
injection

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Types of Adjuvants
1. Alum
2. Oil based
3. Synthetic
Antigenic determinants (epitopes)
Immune cell do not interact with or recognize an entire immunogen molecule instead,
lymphocyte recognize discrete site on macromolecules called epitopes.
B cells recognize conformational, linear T cells
One epitope 5000 dalton
Immunodominant
Hapten
Small molecules or chemical groups that can function as epitope when bound to other carrier
larger molecules are called haptens.

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CHAPTER 4
ANTIBODIES
Antibodies are the antigen binding glycoproteins present on B cell membrane & secreted by
plasma cells.
Basic structure of antibodies
All antibodies have the same basic four poypeptide chain unit: two light (L) chains and two
heavy (H) chains. In the basic unit, one L-chain is bound by disulfide bond to one H chain. The
two arms that bind antigen are termed antigen binding fragments (Fabs), and the rest of the
molecule is the crystallizable fragment (Fc). The molecule can be split by papain to yield two
identical fragments, each with a single combining site for antigen (Fab; fragment antigen
binding), and a third fragment which lacks the ability to bind antigen and is termed Fc (fragment
crystallizable). Pepsin strikes at a different point and cleaves the Fc from the remainder of the
molecule to leave a large fragment which is known as F(ab')2. There are five different kinds of H
chains (referred to as µ, δ, γ, ε, and α chains), which determine the class of antibody (IgG, IgD,
IgG, IgE and IgA respectively).
IgG: It is by far the most prevalent antibody circulating throughout the tissue fluids. It is a
monomer produced by memory cells responding the second time to a given antigenic stimulus. It
neutralizes toxins, opsonizes, and fixes complement and it is the only antibody capable of
crossing the placenta.
IgA: It has two forms (1) a monomer that circulates in small amounts in the blood and (2) a
dimer that is a significant component of the mucous and serous secretions of the salivary glands,
intestine, nasal membrane, udder, lung, and genitourinary tract. The dimer called secretory IgA is
formed in a plasma cell by two monomers attached by a J pieces. To facilitate the transport a
secretory piece is added to IgA.

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IgM: It is huge molecule composed of five monomers (pentamer) attached by the Fc receptors to
a central J chain. This is synthesized first following first encounter with antigen.
IgD: It is a monomer present in very few amounts in serum and it does not fix complement ,
opsonize, or cross the placenta. Its main function is to serve as receptor for antigen on B cells.
IgE: It is a uncommon blood component unless individual suffers from allergic or worm
infections. Its Fc region interacts with receptors on mast cells and basophils. IgE has another
insidious effect- that of mediating anaphylaxis, asthma and certain other allergies.

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Table 2: Difference between different classes of immunoglobulin
IgG
(monomer)
IgA (dimer
only)
(dimer,
monomer)
IgM
(Pentamer)
IgD
Monomer
IgE
(Monomer)
Number of antigen
binding sites
2 4, 2 10 2 2
Molecular weight 150,000 170,000-
385,000
900,000 180,000 200,000
Percent of total
antibody in serum
80% 13% 6% 1% 0.002%
Average life in
serum (days)
23 6 5 3 2.5
Crosses placenta? Yes No No No No
Fixes
complement?
Yes No Yes No No
Fc binds to Phagocytes Phagocytes B
lymphocytes
B
lymphocytes
Mast cells and
basophils
Biological function Long-term
immunity;
memory Abs
Secretory Abs;
on mucous
membranes
Produced at
first
response to
antigen
Receptor on B
cells
Antibody of
allergy; worm
infections
Heavy chain γ α µ δ ε
Largely
synthesized in
Spleen &
lymph nodes
Intestinal &
respiratory
tracts
Spleen &
lymph nodes
Spleen &
lymph nodes
Intestinal &
respiratory
tracts
Half life in days 23 6 5 3 <3
Anaphylactic
hypersensitivity
- - - - ++++
Distribution Extracellular Secretions Extracellular Extracellular Extracellular

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fluid; blood
and lymph
(saliva,
colostrum,
cellular and
blood fluids
fluid; blood
and lymph in
B cell
surface
receptor
fluid; blood
and lymph in
B cell surface
receptor
fluid; blood
and lymph in
mast cell
surface
receptor
Concentration
range in normal
serum
8-16 mg/ml 1.4-4 mg/ml 0.5-0.2
mg/ml
0-0.4 mg/ml 17-450 ng/ml
% Carbohydrate
content
3 8 12 13 12

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Figure 4: Structure of an antibody molecule

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Figure 5: Proteolytic fragments of an IgG molecule

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Table 3: Different classes of immunoglobulin

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CHAPTER 5
CYTOKINES
Cytokines are polypeptides produced in response to microbes and other antigens that mediate
and regulate immune and inflammatory reactions.
Activation phase: In the activation phase of adaptive immune responses, cytokines stimulate the
growth and differentiation of lymphocytes
Effector phase: In the effector phases of innate and adaptive immunity, they activate different
effector cells to eliminate microbes and other antigens
Nomenclature of cytokines
Monokines. Lymphokines, cytokines, interleukin IL (1,2 & so on)

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Figure 6: Functions of cytokines in host defense
Innate immunity
NK cells: IFNγ → Mφ
Mφ: IL12 →NK cells
TNF, IL-1, chemokines → chemotaxis of neutrophil

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Figure 7: Functions of cytokines in host defense

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Adaptive Immunity
APC (ag) → T cells → (IL-12) differentiation & growth of CD4+ T cells
IFNγ → Mφ activation
IL-2 → CD8+ T cells → CTL differentiation
IFNγ + IL-2 & 4 → B cells → Antibody secretion; isotype switching
Properties
Cytokine secretion is a brief, self-limited event
Cytokine actions may be local and systemic.
- autocrine action
- paracrine action
- endocrine action
Cytokines receptors on target cells.
External signals regulate the expression of cytokine receptors and thus the responsiveness
of cells to cytokines. The cellular responses to most cytokines consist of changes in gene
expression and sometimes in the proliferation of the target cells.

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Figure 8: Properties of cytokines

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Table 4: Comparative features of the cytokines of innate and adaptive immunity
Biological action of TNF (Tumor Necrosis Factor)
TNF is the chief mediator of the acute inflammatory response to gram-negative bacteria and
other infectious microbes. It is accountable for many of the systemic complications of acute
infections. The major cellular derivation of TNF is activated mononuclear phagocytes. The chief
physiologic purpose of TNF is to stimulate the mobilisation of neutrophils and monocytes to
sites of infection and to activate these cells to eradicate microbes.
Low quantity: Local inflammation
Moderate quantities: Systemic effect: fever, acute phase protein, bone marrow
High quantity: Septic shock: Low heart output, thrombus, hypoglycemia

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Figure 9: Biologic actions of TNF

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Biological action of IL-12
IL-12 is a principal mediator of the early innate immune response to intracellular microbes and is
a key inducer of cell mediated immunity, the adaptive immune response to these microbes. The
principal sources of IL-12 are activated dendritic cells and macrophages. IL-12 is critical for
initiating a sequence of responses involving macrophages, NK cells, and T lymphocytes that
results in the eradication of intracellular microbes.
Fig 10: Biologic function of IL-12

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Regulation of IL-2 receptor expression
IL-2 is a survival, growth, and differentiation factor for T lymphocytes. It plays a pivotal role in
regulation of T cell response through its actions on regulatory T cells. CD4+
T lymphocytes
chiefly produce IL-2. Activated T cells and costimulators stimulate transcription of the IL-2
gene and synthesis and secretion of the protein.
Interleukin-4
IL-4 is the chief stimulus for the production of IgE antibodies. It is responsible for the
development of TH2 cells a subset of naive CD4+
helper T cells.
Biologic actions
IL-4 is the major cytokine that provokes B cell immunoglobulin heavy chain class switching to
the IgE isotype. It stimulates the development of TH2 cells from naive CD4+
T cells and
functions as an autocrine growth factor for differentiated TH2 cells.
IFNγ
IFN-γ is the principal macrophage activating cytokine and serves critical functions in innate
immunity and in adaptive cell mediated immunity against intracellular microbes.
Biologic actions
IFN-γ activates macrophages to kill phagocytosed microbes.
IFN-γ promotes the differentiation of naïve CD4+
T cells to the TH1 subset and inhibits the
differentiation of TH2 cells.
IFN-γ acts on B cells to promote switching to certain IgG subclasses, notably IgG2a in mice.
IFN-γ stimulates expression of class I and class II MHC molecules and costimulators on APCs.
Transforming growth factor – β
The principal action of TGF-β in the immune system is to inhibit the proliferation and activation
of lymphocytes and other leukocytes.
Biologic actions
TGF-β inhibits the proliferation and effector functions of T cells and the activation of
macrophages.
TGF-β regulates differentiation of functionally distinct subsets of T cells.
TGF-β stimulates production of IgA antibodies by inducing B cells to switch to this isotype.

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CHAPTER 6
LYMPHOID ORGAN
Primary Lymphoid Organ (Generative)
Lymphoid tissues are classified as generative organs, also called primary lymphoid organs,
where lymphocytes first express antigen receptors and attain phenotypic functional maturity.
Peripheral lymphoid organs / secondary lymphoid organs
Where lymphocyte responses to foreign antigen are initiated and develop.
Primary: Bone marrow, thymus, Bursa of Fabricius
Fig 11: Histology of bursa of Fabricius

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Source: Ian R. Tizard: Veterinary Immunology
Bone marrow
The bone marrow is the site of generation of all circulating blood cells in the adult, including
immature lymphocytes and is the site of early events in B cell maturation. Bone marrow are off 2
types 1. red marrow (consisting mainly of hematopoietic tissue) and 2. yellow marrow
(consisting mainly of fat cells. Red marrow is found mainly in the flat bones, such as the breast
bone, cranium, hip bone, vertebrae, ribs, and shoulder blades, and at the epiphyseal ends of long
bones such as the humerus and femur. Yellow marrow is found in the medullary cavity, the
hollow interior of the middle portion of long bones.

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Fig 12: Thymus
Site for T cell maturation
The thymus is the site of T cell maturation. The thymus is bilobed organ situated in the
mediastinum. Each lobe is divided into multiple lobules by fibrous septa and each lobule consists
of an outer cortex and an inner medulla. The cortex contains a dense collection of T lymphocytes
and the lighter staining medulla is more sparsely populated with lymphocytes. In the medulla
Hassal’s corpuscle is present. It is composed of tightly packed whorls of epithelial cells that may

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be remnants of degenerating cells. The thymus has a rich in blood supply and efferent
lymphatic vessels that drain into mediastanal lymph nodes. Only mature T cells exit the thymus
and enter the blood and peripheral lymphoid tissues.
Bursa of Fabricius
Round sac located just above the cloaca. Greatest size attain within 1-2 week. With the
advancement of the age it involutes. In mature birds it is difficult to identify.
Structure: Like thymus lymphocytes are embedded in epithelial tissues. Fold of epithelium
extend into the lumen of sac.
Lymphoid follicle are scattered in the fold
Cortex & Medulla
Cortex → Lymhpocytes, plasma cells, Mφ
Medulla → epithelial cells which are replaced by lymphoblast & lymphocytes
Dendritic cells → surround follicle
Function
Maturation & differentiation of B cells
Bursa acts like thymus → immature cells produced in bone marrow migrate to the bursa. Then
these cells proliferate rapidly 90-95% die due to apoptosis.
It can also trap antigen and antibody synthesis takes place against the antigen.
It secretes hormone bursin which activate B cells.

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Peripheral lymphoid organs
Lymph Nodes
Lymph nodes are interposed along lymphatic vessels. A lymph node consists of outer cortex and
inner medulla. It is covered by fibrous capsule. Afferent lymph vessels bring lymph into cortex.
By efferent lymph vessel lymph is drain out. The anatomic segregation of different classes of
lymphocytes in distinct areas of the node is dependent on cytokines.
Cortex = follicles → central area (germinal centre)
(B cell zone) Primary follicle Secondary follicle
Para cortex : Reticular fibers, dendritic cells, Mφ
Medulla : T cell zone
Primary follicle: Mature, naive B cell
Follicular dendritic cell → germinal centre

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Lymph nodes & lymphatic
system
Fig 13: Schematic diagram of a lymph node illustrating the T cell rich and B cell rich zones
and the routes of entry of lymphocytes and antigen (shown captured by dendritic cell)

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Fig 14: Schematic diagram illustrates the path by which naive T and B lymphocytes
migrate to different areas of lymph node
Antigens are transported to lymph node mainly in lymphatic vessels. The skin, epithelia &
parenchymal organs contain numerous lymphatic capillaries that absorb and drain fluid from
spaces between tissue cells. The absorbed interstitial fluid called lymph flows through the
lymphatic capillaries into convergent. These vessels merge into afferent lymph vessels that drain
into the subscapsular sinuses of lymph nodes. The efferent lymph vessel at the end of a lymph
node chain joins other lymph vessels, eventually culminating into one large lymphatic vessel
called thoracic duct. Lymph from the thoracic duct is emptied into the superior vena cava, thus
returning the fluid to the blood stream.

46
Fig 15: Spleen
The spleen is the major site of immune responses to blood borne antigens. It is supplied by single
splenic artery which pierce the capsule at the hilum and divides into progressively smaller
branches. The lymphocytes rich regions of the spleen called white pulp are organized around
branches of these arteries called central arteries. Central arteriole are surrounded by cuffs of
lymphocytes, most of which are T cells. zone called PALS (periarteriolar lymphoid sheath).
Follicular arteriole drain into marginal sinus. B cells rich follicles occupy the space between the
marginal sinus & PALS.
Marginal zone: Outside the marginal sinus there is a distinct region called region called
marginal zone ( B cell + Mφ ) which forms the outer boundary of white pulp.
White pulp
Red pulp: Some arteriolar branches end in extensive vascular sinusoids, scattered among these
are RBCs, Mφ, dendritic cell, lymphocytes, plasma cells
Filter: Microbes, RBCs
Red pulp macrophages clear microbes and damaged RBCs. The spleen is the major site for
phagocytosis of antibody coated (opsonized) microbes. Individuals lacking a spleen are highly
susceptible to infections with encapsulated bacteria such as pneumococci.

47
CHAPTER 7
PHAGOCYTOSIS
Phagocytosis: Second line of defense. Ingestion of microorganism & other particles (phage: eat
cyte: cell, osis: process). Cells: Phagocytes (neutrophils & macrophages).
Activation: defensive cells→ blood → bone marrow (myelos, myeloid stem cells origin)
Neutrophil: rapid but lack sustained effect
Macrophages : Sentinel cells, repeated phagocytois : sustained effect. Neutrophil : Polymorpho
nuclear (PMN) cells, granulocytes. Neutrophils are formed in bone marrow. They migrate to
blood stream and about 12 hours later move into the tissue. There are 2 pools of neutrophils in
blood: a circulating pool and a pool of cells sequestered in capillaries. During bacterial infection
neutrophil increases 10 fold as they are released from the bone marrow and the sequestered pool.
Emigration of neutrophil from blood vessels due to (1) changes in endothelium (2) neutrophil,
1.Changes in endothelium
Lipopolysaccharide, thrombin and histamine express E/P Selectin receptor on endothelial cells
which cause appearance of E or P selectin ligand in neutrophils. This will slow down neutrophil
movement.
2.Changes in neutrophil
As the neutrophils roll along the endothelial surface, the second stage of adhesion occurs. A lipid
called PAF( platelet activating factor) secreted by endothelial cells, activates the rolling
neutrophils. As a result, the neutrophil express protein called LFA-1. LFA-1(leukocyte function
associated antigen) binds strongly with ICAM-1 (intercellular adhesion molecule) expressed on
inflamed endothelial cells. This strong binding makes the neutrophil come to a complete stop and
attaches it firmly to blood vessel wall.

48
Emigration
The neutrophils squeeze between the endothelial cells and the basement membrane. Since
neutrophils are the most mobile of all blood leukocyte. They are the first cells to arrive at the
damaged tissues.
Fig 16: Migration of naive and effector T lymphocyte
Phagocytosis can be divided into (1) Activation (2) Chemotaxis (3) Adherence (4) Ingestion (5)
Digestion.

49
Fig 17: Phases of phagocytosis
Source: Tortora, Funke and Case: Microbiology
Activation: TNF α activates neutrophils → degranulates mount a respiratory burst & release
elastases, defensins & oxidants which attract more neutrophils.
Chemotaxis: The directed migration of neutrophils is called chemotaxis. Chemotaxis molecules
are microbial products, components of WBC, damaged tissue cells, peptides derived from
complement.
Adherence: Neutrophil + Bacteria → - ve charge repel each & suspended in fluid → +ve charge
molecules called opsonin coat bacteria → e.g. mannose binding lectin (MBL) + complement
components.
(Type I phagocytosis) Antibody receptor mediated phagocytosis
(Type II phagocytosis complement mediated) most effective opsonins → antibody
Surface phagocytosis : Bacteria lagged against some surface
Ingestion
When neutrophils meets a bacterium its pseudopodia flows over & around it then drawn into cell
in vacuole called phagosome. Ingestion may or may not be dependent on opsonization. Mannose
receptor or integrin can bind directly to bacteria.

50
Destruction
There are two distinct processes (1) Respiratory burst (generation of potent oxidants ) (2)
Release of lytic enzymes & antimicrobial peptides from intracellular granules. Formation of
phagosome. Membrane enzymes → pump proton H + into phagosome lower pH 4 → hydrolytic
enzymes are activated.
Digestion
Phagosome + lysosome (digestive enzyme + bactericidal substances) → phagolysosome, 10-30
minutes killing,
1. Lysozyme → hydrolytic enzyme peptidoglycan
2. Lipase
3. Protease
4. Ribonuclease
5. Deoxyribonuclease & antimicrobial peptides like defensins
Lysosome also contain enzyme that produce toxic oxygen products (phagocyte oxidase 100 time
O2 consumption) → RoS (reactive oxygen species)
1. Superoxide radical O2
-
.
2. H2O2
3. Singlet oxygen (1
O2
-
)
4. OH-
(oxidative respiratory burst) other enzyme use these products → killing of microbes
e.g. myeloperoxidase → Cl-
+ H2O2 → HOCl highly toxic bleach.
Residual body → discharge wastes outside the cell
1. Respiratory burst (RoS) 2. Hydrolytic enzymes (lysozymes, lipase, protease,
ribonuclease, deoxyribonuclease) 3. Defensin 4. Reactive nitrogen intermediates (NO)
2. Respiratory burst (sometimes called oxidative burst) is the quick release of reactive
oxygen species (superoxide radical and hydrogen peroxide) from different types of cells.
Usually these chemicals are released from immuno competent cells, e.g., neutrophils and
monocytes, when they come into contact with various fungi or bacteria. Respiratory
burst plays a crucial role in the immune system. It is an important reaction that occurs in
phagocytes to degrade internalized bacteria and particles.

51
Microbial evasion of phagocytosis
Structure that inhibit adherence →capsule + M protein. e.g. Streptococcus pyogenes ( m protein)
Ingested microbes release substance which kill phagocytes e.g. leukocidin by Staph. Streptolysin
by streptococcus.
Some bacteria secrete MAC (microbial attack complex) → lyse phagocyte cell. e.g.Listeria
monocytogenes.
Some microorganism have ability to survive inside phagocyte, listeria, shigella, rickettsia escape
phagosome before fusion with lysosome.
Mycobacterium & HIV prevent fusion and acidification. Some remain dormant in it like
Brucellosis.

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CHAPTER 8
COMPLEMENT
Complement system: These are series of protein synthesized in liver and found in serum, tissues.
They are 30 in number. They destroy microbes by following ways
1. Cytolysis
2. Inflammation
3. Through opsonization

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Fig 18: Outcomes of complement activation

54
Fig 19: Classical pathway of complement activation

55
Fig 20: Alternative pathway of complement activation

56
Fig 21: Lectin pathway of complement activation
3 pathways
Classical Alternative MBL
Ag + Ab Microbe Microbe
C1
C2
C3
B , D P
C3 C2 C4

57
CHAPTER 9
HYBRIDOMA & MONOCLONAL ANTIBODY
Myeloma cells can be grown in tissue culture, where they survive indefinitely
It would be highly desirable to be able to set up a system to obtain large quantities of
absolutely pure, specific immunoglobulins directed against an antigen of interest
This can be done by fusing a normal plasma cell, making the antibody of interest, with a
myeloma cell able to grow in tissue culture
The resulting mixed cell is called hybridoma
The first stage in making a hybridoma is to generate antibody-producing plasma cells
This is done by immunizing a mouse against the antigen of interest and repeating the process
several times to ensure that a good antibody response is mounted
Two to four days after the antigen is administered, its spleen is removed and broken up to
form a cell suspension
These spleen cells are suspended in culture medium, together with cultured mouse
myeloma-cells
Generally, myeloma cells that do not secrete immunoglobulins are used, since this
simplifies purification later on
Polyethylene glycol is added to the mixture
This compound induces many of the cells to fuse (it takes about 200,000 spleen cells on
average to form viable hybrid with one myeloma cell)
If the fused cell mixture is cultured for several days, any un-fused spleen cells will die
The myeloma cells would normally survive, but they are eliminated by a simple trick
There are three biosynthetic pathways by which cells can synthesize nucleotides and
therefore nucleic acids
The myeloma cells are selected so that they lack two enzymes: hypoxanthine
phosphoribosyl transferase and thymidine kinase
As a result, they cannot use either thymidine or hypoxanthine and are obliged to use an
alternative biosynthetic pathway to convert uridine to nucleotides

58
The fused cell mixture is therefore grown in a culture containing three compounds:
hypoxanthine, aminopterin, and thymine (known as HAT medium)
Aminopterin is a drug that prevents cells from making their own nucleotides from uridine
Since the myeloma cells cannot use hypoxanthine or thymidine and the aminopterin stops
them from using the alternative synthetic pathway, they cannot make nucleic acids and
soon die
Hybrids made from a myeloma and a normal cell will grow, since they possess the
critical enzymes and can therefore utilize the hypoxanthine and thymidine in the culture
medium and survive
The hybridomas divide rapidly in the HAT medium, doubling their numbers every 24 to
48 hours
On average, about 300-500 different hybrids can be isolated from a mouse spleen,
although not all will make antibodies of interest
If a mixture of cells from a fusion experiment is cultured in wells on a plate with about
50,000 myeloma cells per well, it is usual to obtain about one hybrid in every three wells
After culturing for 2-4 weeks, the growing cells can be seen and the supernatant solution
can be screened for the presence of antibodies
It is essential to use sensitive assay at this time (ELISAs)
Clones that produce the desired antibody are grown in mass culture and recloned to
eliminate non antibody producing hybrids
Unfortunately, antibody procucing clones tend to lose this ability after being cultured for
several months
Thus it is usual to make large stocks of hybridoma cells and store them frozen in small
aliquots
These can then be thawed as required and grown up in bulk culture
Alternatively, the hybridoma cells can be injected intraperitoneally into mice
Since they are tumor cells, the hybridomas grow rapidly and provoke the effusion of
large volume of fluid into the mouse peritoneal cavity
This fluid is rich in monoclonal antibodies and can be readily harvested
1. HAMA (Human anti murine antibody response)

59
Fig 22: A schematic diagram showing the method of production of monoclonal antibodies

60
Fig 23: The pathways of purine synthesis and the mechanism of action of HAT medium
Source: Ian R Tizard: Veterinary Immunology

61
CHAPTER 10
MAJOR HISTOCOMPATIBILITY COMPLEX
Locus of genes for MHC molecules
Two types of products 1. Class I MHC molecule (CD8 + CTLs)
2. Class II MHC molecule (CD4 + helper T cells)
Protein ag 1. Intracellular or 2. Extracellular
HLA (Human leukocyte antigen) are also called Human MHC molecule - Mouse H2 (Immune
response gene, IR)
Figure 24: Structure of a class I MHC molecule

62
Figure 25: Structure of a class II MHC molecule
Traits Class I MHC molecule Class II MHC molecule
Location of polymorphic
residue
α1 α2 α1 β1
Binding site for T cell co-
receptor
α3 + CD8 β2 +CD4
Size of peptide binding cleft 8-11 10-30
Close cleft Open cleft
Availability All nucleated cells Dendritic cells, macrophage, B
cells,
IFNγ increase expression of MHC II molecule (NK, CD4 T cells)

63
CHAPTER 11
ANTIGEN PROCESSING AND PRESENTATION
Cells that display MHC associate peptides are called APCs (antigen presenting cell).
Certain APCs present antigens to naive T cells during the recognition phase of immune
responses to initiate these responses.
Some APCs present antigens to differentiated T cells during the effector phase to trigger the
mechanisms that eliminate the antigens.
Most T lymphocytes recognize only peptides, whereas B cells can recognize peptides, proteins,
nucleic acids, polysaccharides, lipids, and small chemicals.
Hapten-conjugated peptides are recognized by T cells.
T cells recognize linear and not conformational determinants of peptide antigens . In contrast, B
cells may recognize conformational determinants that exist when antigens, such as globular
proteins, are in their native tertiary (folded) configuration.
Self MHC restriction: T cells from any one individual recognize foreign peptide antigens only
when these peptides are bound to and displayed by the MHC molecules of that individual. This
feature of antigen recognition by T cells, called self MHC restriction.
Lipid ag presentation: The class I-like nonpolymorphic molecule CD1 is expressed on a variety
of APCs and epithelia, and it presents lipid antigens to unusual populations of non-MHC-
restricted T cells. A variety of cells can recognize lipid antigens presented by CD1; these include
CD4+, CD8+, and CD4-CD8- T cells.
Antigen processing: The conversion of native proteins to MHC-associated peptide fragments by
APCs is called antigen processing.
Costimulator: Some APCs provide stimuli to the T cell beyond those initiated by recognition of
peptide-MHC complexes by the T cell antigen receptor. These stimuli, referred to as
costimulators.
Adjuvant : Adjuvants promote T cell activation by several mechanisms.
1. Local inflammation and thus stimulate the influx of APCs to sites of antigen exposure.
2. Adjuvants activate APCs to increase the expression of costimulators and to produce soluble
proteins, called cytokines, that stimulate T cell responses.

64
3. Some adjuvants may also act on APCs to prolong the persistence of peptide-MHC complexes
on the cell surface
Types of APCs: Dendritic (naive), macrophage differentiation effector CD4+, B cells (effector T
cells)
Dendritic cells: are present in lymphoid organ, in the epithelia of skin, gastrointestinal and
respiratory tracts and in the interstitium of most parenchymal organ. These cells are identified by
having spine like projections. All dendritic cells are thought to arise from bone marrow precursor
called myeloid dendritic cell. The prototypes of epithelial dendritic cells are the Langerhans cells
of the epidermis. Because of their long cytoplasmic processes, Langerhans cells occupy as much
as 25% of the surface area of the epidermis even though they constitute less than 1% of the cell
population.
Dendritic cells are most effective because of
1. Dendritic cells are strategically located at the common sites of entry of microbes and foreign
antigens.
2. Dendritic cells express receptors that enable them to capture microbes.
3. These cells migrate preferentially to the T cell zones of lymph nodes, through which naive T
lymphocytes circulate searching for foreign antigens
4. Mature dendritic cells express costimulators, which are needed to activate naive T cells.
B cells: Internalize soluble protein antigens and present processed peptides derived from these
proteins to helper T cells. The antigen-presenting function of B cells is essential for helper T
cell-dependent antibody production.
Macrophage: Low level of MHC II expression, γ IFN increase expression of MHC II
Receptor mediated endocytosis
Other Antigen Presenting Cells (APCs)
1. Vascular endothelial cells : class II molecules
2. Epithelia & mesenchymal cells: by γ IFN
Antigen: that enter the blood stream are captured by antigen presenting cells in the spleen.
Dendritic cells : that are activated by microbes and by locally produced cytokines, such as tumor
necrosis factor, lose their adhesiveness for epithelia and begin to express a chemokine receptor
called CCR7 that is specific for chemokine produced in T cell zones of lymph nodes. The

65
chemokines attract the dendritic cells bearing microbial antigens into the T cell zones of the
regional lymph nodes.
Presentation of Protein Antigens to CD8+ T Lymphocytes
All nucleated cells can present class I MHC-associated peptides, derived from cytosolic protein
antigens, to CD8+
T lymphocytes
Figure 26: Cross-presentation of antigens to CD8+ T cells
Cells infected with intracellular microbes, such as viruses are ingested by dendritic cells and the
antigens of the infectious microbes are processed and presented in association class I MHC
molecules to CD8+
T cells. Thus dendritic cells are able to present endocytosed vesicular
antigens by the class I pathway. At the same time, the dendritic cells can present class II MHC
associated peptides generated in the vesicles to CD4+ cells. This process is called cross-
presentation to indicate that one cell type (the dendritic cell) can present antigens from another
cell and activate T cells specific for these antigens.
Pathways of ag processing & presentation
The pathways of antigen processing convert protein antigens derived from the extracellular
space or the cytosol into peptides and load these peptides onto MHC molecules for display to T
lymphocytes.
Processing of endocytosed antigens for class II MHC associated presentation
Most class II associated peptides are originated from protein antigens that are seized and
internalized into endosomes by unique APCs. Internalized proteins are degraded enzymatically
in lysosome and endosomes to generate peptides that are able to attach to the peptide-binding

66
groove of class II MHC molecules. Class II MHC molecules are manufactured in the
endoplasmic reticulum and transported to endosome with an associated protein called the
invariant chain (Ii) which resides in the peptide-binding clefts of the newly synthesized class II
molecules. Within the MIIC the Ii is removed from class II MHC molecules by the integrated
action of HLA-DM molecules and proteolytic enzymes. The antigenic peptides then able to
attach to the available space in peptide-binding clefts of the class II molecules. Class II MHC
molecules are stabilized by the bound peptides. The stable peptide class II complexes are
delivered to the surface of the APC, where they are presented for recognition by CD4+
T cells.
Fig 27: The class II MHC pathway of antigen processing

67
Processing of cytosolic antigens for Class I MHC associated presentation
1 .SOURCES OF CYTOSOLIC ANTIGENS
The peptides that are presented bound to class I MHC molecules are derived from cytosolic
proteins, most of which are endogenously synthesized in nucleated cells.
2. PROTEOLYTIC DEGRADATION OF CYTOSOLIC PROTEINS
The major mechanism for the generation of peptides from cytosolic protein antigens is
proteolysis by the proteasome.
3.TRANSPORT OF PEPTIDES FROM THE CYTOSOL TO THE ENDOPLASMIC
RETICULUM
Peptides generated in the cytosol are translocated by a specialized transported into the ER, where
newly synthesized class I MHC molecules are available to bind the peptides.
4.ASSEMBLY OF PEPTIDE-CLASS I MHC COMPLEXES IN THE ER
Peptides translocated into the ER bind to class I MHC molecules that are attached to the
transporter associated with antigen processing (TAP) dimer.
5.SURFACE EXPRESSION OF PEPTIDE-CLASS I MHC COMPLEXES
Class I MHC molecules with bound peptides are structurally stable and are expressed on the cell
surface.

68
Fig 28: The class I MHC pathway of antigen presentation

69
CHAPTER 12
ANTIGEN RECEPTORS AND ACCESSORY MOLECULES OF T
LYMPHOCYTES
T- Cell receptors
TCR: The TCR is a clonally distributed receptor, meaning that clones of T cells with different
specificities express different TCRs.
TCR complex: Antigen recognition are transduced not by the TCR itself but by invariant
proteins called CD3 and ζ, which are noncovalently linked to the antigen receptor to form the
TCR complex.
Accessory molecule
Other membrane receptors that do not recognize antigen but participate in responses to antigens.
TCR (co receptor for ag) + CD3 & Zeta (signal) + CD4 or CD8 (self MHC recognition) + CD28
(B-7 costimulator) + integrin (adhesin)

70
Figure 29: T cell receptors and accessory molecules

71
Fig 30: Structure of T cell Receptor
CD8+
& CD4+
→ heterodimer → 2 chain α, β covalently linked with disulphide bond. 2 domains
variable domain & constant domain. Number of CDRs: 3 in each 4th
in β
CD3 & Zeta
Non covalently linked to TCR: Both chains contain ITAM : Immunoreceptor tyrosine based
activation motif. Transduce signal that activates T- cells.
Co-receptor (CD4 & CD8)
Enhance signaling → MHC
Co-stimulatory receptors
MHC complex – X, Other molecules on
APC 1. CD28- B7 (1-2) 2. CD2- LFA3 3. LFA-I → ICAM – I
Distribution of lymphocyte
65% CD4+ 35% CD8+

72
Figure 31: The interaction of a CD4+ helper T cell with an APC (A) involves multiple T cell

73
Figure 32: The interaction a CD8+ CTL with a target cell (B), involves multiple T cell
Regulation of integrin avidity
Chemokines increase avidity
Activation of T-lymphocytes (Differentiation)
Naive T lymphocytes recognize antigens and are activated in peripheral lymphoid organs
Effector T cells recognize antigens in lymphoid organs or in peripheral nonlymphoid tissues and
perform their effector functions
The activation of T cells requires recognition of antigens displayed on APCs, costimulators
("second signals"), and cytokines produced by the APCs and by the T cells themselves
Naive T cells require activation by dendritic cells, whereas effector T cells can respond to
antigens presented by a wider variety of APCs.
Functional response of T cells
Secretion of Cytokines
Proliferation (autocrine growth pathway)
Differentiation into Effector Cells (active but short-lived)

74
Differentiation into Memory Cells (quiescent but long-lived)
Decline of T Cell Responses.
Role of Costimulators in T Cell Activation
The best characterized costimulatory pathway in T cell activation involves the T cell surface
molecule CD28, which binds the costimulatory molecules B7-1 (CD80) and B7-2 (CD86)
expressed on activated APCs
Figure 33: Functions of costimulators in T cell activation
Figure 34: Role of B7 and CD28 in T cell activation

75
Signal transduction by TCR complex
The goal of T cell signaling pathways is to coordinately activate the transcription of genes that
are silent in naive cells and whose products mediate the responses and functions of activated T
cells Goal of signal transduction activate transcription of genes ( which were silent in naïve T –
cells)
Role of CD 40 in T- cell activation
Naive T cell → Ag + B7 → induces the expression of CD40 ligand on activated T cell → it
engages CD40 molecule on APC → may stimulate expression of more B7 molecules on APCs

76
Figure 38: Role of CD40 in T cell activation
Formation of immunological synapse
When the TCR complex recognizes MHC-associated peptides on an APC, several T cell
surface proteins and intracellular signaling molecules are rapidly mobilized to the site of
T cell-APC contact.
This region of physical contact between the T cell and the APC has been called the
immunological synapse.
cSMAC (central supramolecular activation cluster)
pSMAC (peripheral supramolecular activation cluster)

77
Figure 39: Formation of the immunological synapse

78
CHAPTER 13
B CELL ACTIVATION AND ANTIBODY PRODUCTION
General Features of Humoral Immune Responses
The process of activation of B cells and the generation of antibody-producing cells consists of
sequential phases.
Antibody responses to protein antigens require CD4+ helper T lymphocytes that recognize the
antigen and play an essential role in activating B lymphocytes.
Antibody responses to nonprotein antigens, such as polysaccharides and lipids, do not require
antigen-specific helper T lymphocytes
Activated B cells differentiate into antibody-secreting cells, some of which continue to produce
antibodies for long periods, and into long-lived memory cells.
Heavy chain isotype switching and affinity maturation are typically seen in helper T cell-
dependent humoral immune responses to protein antigens.
Primary and secondary antibody responses to protein antigens differ qualitatively and
quantitatively.
Figure 40: Phases of the humoral immune response

79
Figure 41: Kinetics of primary and secondary humoral immune responses
Table 5: Features of Primary and Secondary Antibody Responses
B cell receptor
B cell antigen receptor complex

80
- Membrane IgM & IgD naive B –cell non covalent associated with invariant Igα Igβ
molecule which contain ITAM for signal transduction.
Fig 42: Signal transduction
B cells express a complex of the CR2 complement receptor, CD19 and CD81. Microbial antigens
that have bound the complement fragment C3d can simultaneously engage both surface of a B
cell. This leads to the initiation of signaling cascades from both the BCR complex and the CR2
complex, because of which the response to C3d antigen complexes is greatly enhanced compared
with the response to antigen alone.

81
Fig 43: Role of complement in B cell activation
B cells express a complex of the CR2 complement receptor, CD19 and CD81. Microbial
antigens that have bound the complement fragment C3d can simultaneously engage both the
CR2 molecule and the membrane Ig on the surface of a B cell. This leads to the initiation of
signaling cascades from both the BCR complex and the CR2 complex, because of which the
response to C3d antigen complexes is greatly enhanced compared with the response to
antigen alone.
The importance of the complement system in humoral immune response has been
established by several experiments.
1. If C3d is covalently attached to a protein antigen, the modified antigen is about 1000 fold
more immunogenic than the native antigen.
2. Knockouts of the C3, CR2 or CD19 genes in mice results in defects in antibody
production.

82
Functional response of B cells to Ag recognition
The early cellular events that are induced by antigen mediated cross-linking of the BCR
complex initiate B cell proliferation and differentiation and prepare the cells for subsequent
interactions with helper T cells.
Figure 44: Functional responses induced by antigen-mediated cross-linking of the BCR
complex
Antigen mediated cross linking of the B cell antigen receptor induces several cellular
responses, including mitosis, expression of new surface molecules, including costimulators

83
and cytokine receptors, and altered migration of the cells as a result of the expression of
CCR7.
HELPER T CELL DEPENDENT ANTIBODY RESPONSE TO PROTEIN
ANTIGENS
Antibody response to protein antigens require recognition of the antigen by helper T cells and
cooperation between the antigen specific B and T lymphocyte.
The sequence of events in T cell dependent antibody responses
1. Antigen is taken up by dendritic cells (DCs) and presented to helper T cells
2. Helper T cells are activated and induced to express membrane proteins (CD40L) and
cytokines
3. Activated helper T cells are instructed to migrate toward the follicle following a
chemokine gradient.
4. B cells are activated by antigen that may be in soluble form or displayed by DCs.
5. B cells process and present antigen, alter their cell surface chemokine receptor profile,
and migrate toward the T cell zone.
6. T and B cells interact at the T-B interface, and B cells are activated by CD40L and
cytokines.
7. Small extrafollicular B cell foci form in T cell zones, and some isotype switching and Ig
secretion occur.
8. Activated B cells migrate back into the follicle. Germinal centres within the follicles and
are sites of extensive isotype switching, somatic mutation, the selection events that lead
to affinity maturation and memory B cell generation.
9. Long lived plasma cells are generated from cells activated in the germinal center reaction,
and some of these terminally differentiated plasma cells migrate to the bone marrow.

84
Fig 45: Early and late events in humoral immune response to T cell dependent protein
antigens
Immune responses are initiated by the recognition of antigens by B cells and helper T cells.
The activated lymphocytes migrate toward one another and interact, resulting in B cell
proliferation, differentiation into antibody secreting cells and early isotype switching. The
late events occur in germinal centers and include affinity maturation of the response and
additional isotype switching and memory B cell generation.
The Germinal Center Reaction
A number of events that are characteristic of helper T cell dependent antibody response,
including affinity maturation, isotype switching and the generation of memory B cells, occur
primarily in the germinal centres of lymphoid follicle. In response to CD40 engagement and
cytokines, some of the progeny of activated IgM and IgD expressing B cells undergo the

85
process of heavy chain isotype (class) switching, leading to the production of antibodies with
heavy chains of different classes, such as γ, α, ε.
Fig 46: Germinal centre reactions in T – Cells dependent response
B cells that have been activated by helper T cells at the edge of a primary follicle migrate
into the follicle and proliferate, forming the dark zone. Somatic mutations of Ig V genes
occur in these B cells, and they migrate into the light zone where they encounter follicular
dendritic cells displaying antigen. B cells with the highest affinity Ig receptors are selected to
survive, and they differentiate into antibody secreting or memory B cells.
ANTIBODY FEED BACK: REGULATION OF HUMORAL IMMUNE RESPONSES
BY FC RECEPTORS
Secreted antibodies inhibit continuing B cell activation by forming antigen-antibody
complexes that simultaneously bind to antigen receptors and Fcγ receptors on antigen

86
specific B cells. This is the explanation for a phenomenon called antibody feed back which
refers to the down regulation of antibody production by secreted IgG antibodies.
Figure 47: Regulation of B cell activation by Ig Fc receptors
Antigen antibody complexes can simultaneously bind to membrane Ig (through antigen) and
the FcγRIIB receptor through the Fc portion of the antibody. As a consequence of this
simultaneous ligation of receptors, phosphatases associated with the cytoplasmic tail of the
FcγRIIB inhibit signaling by BCR complex and block B cell activation.

87
CHAPTER 14
IMMUNOLOGICAL TOLERANCE
Immunological tolerance is defined as unresponsiveness to an antigen.
Importance
Normal individuals are tolerant of their own antigens (self antigens) because the lymphocytes
that recognize self antigens are killed or inactivated or change their specifity. Foreign antigens
may be administered in ways that inhibit immune responses by inducing tolerance in specific
lymphocyte. The induction of immunological tolerance may be exploited as a therapeutic
approach for preventing harmful immune response
Fates of lymphocytes after encounter with antigen
1. Lymphocyte + ag → proliferation (normal)
2. Lymphocyte + tolerogenic ag → death, anergy + immunogenic ag → no response
(Central & peripheral tolerance to self ag)
Figure 48: Fates of lymphocytes after encounter with antigens

88
GENERAL FEATURES AND MECHANISM OF IMMUNOLOGICAL TOLERANCE
Tolerance is described as unresponsiveness to an antigen that is provoked by previous exposure
to that antigen. Self tolerance is produced by exposing own antigen in immature self reactive
lymphocytes in generative lymphoid organs (central tolerance) or in mature lymphocytes in
peripheral sites (peripheral tolerance). Central tolerance occurs because during their maturation
in the generative lymphoid organs, all lymphocytes pass through a stage in which encounter with
antigens leads to cell death or the expression of new antigen receptors or a change in functional
capabilities.
Peripheral tolerance occurs when mature lymphocytes that recognize self antigens become
incapable of responding to that antigen or lose their viability and become short lived cells or are
induced to die by apoptosis.
T LYMPHOCYTE TOLERANCE
Central tolerance in T cells
During their maturation in the thymus, many immature T cells that recognize antigens with high
avidity are deleted. Some self reactive CD4+ T cells that see self antigens in the thymus are not
deleted but instead differentiate into regulatory T cells.
Figure 49: Central T cell tolerance

89
Peripheral T cell Tolerance
Peripheral tolerance is the mechanism by which mature T cells that recognize self antigens in
peripheral tissues become incapable of subsequently responding to these antigens.
Anergy (Functional Unresponsiveness) Induced by Recognition of Self Antigen
Exposure of CD4+
T cells to an antigen in the absence of costimulation or innate immunity may
make the cells incapable of responding to that antigen. Anergy results from biochemical or
genetic alterations that reduce the ability of lymphocytes to respond to self antigens
Figure 50: T cell anergy induced by a self antigen in transgenic mice

90
Figure 51: T cell anergy
Suppression of self reactive lymphocytes by regulatory T cells
Regulatory T lymphocytes are a subset of CD4+ T cells whose function is to suppress immune
response and maintain self-tolerance.
Deletion of T Cells by Apoptotic Cell Death
T lymphocytes that recognize self antigens without inflammation or that are repeatedly
stimulated by antigens die by apoptosis. T cells that recognize self antigens without
costimulation or an accompanying innate immune response may activate a pro-apoptic protein
called Bim, resulting in apoptosis by the mitochondrial pathway. Repeated stimulation of T cells
results in the co-expression of death receptors and their ligands, and engagement of the death
receptors triggers apoptotic death.

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Figure 52: Activation-induced death of T lymphocytes

92
Fig 53: Biochemical mechanisms of apoptosis

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B LYMPHOCYTE TOLERANCE
Central Tolerance in B Cells
Immature B lymphocytes that recognize self antigens in the bone marrow with high affinity
either change their specifity or are deleted.
Peripheral B Cell Tolerance
Mature B lymphocytes that recognize self antigens in peripheral tissues in the absence of specific
helper T cells may be rendered functionally unresponsive or die by apoptosis.
Figure 54: Central tolerance in B lymphocytes in a transgenic mouse model

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Figure 55: Peripheral B cell tolerance in a transgenic mouse model

95
Table 5: Self Tolerance in T and B Lymphocyte
Feature T Lymphocytes B lymphocytes
Principal sites of tolerance
induction
Thymus (cortex) ; Periphery Bone marrow; periphery
Tolerance-sensitive stage of
mutation
CD4+CD8+ (double positive)
thymocyte
Immature (IgM+IgD-)
lymphocyte
Stimuli for tolerance induction Central: high avidity
recognition of antigen in
thymus
Central: recognition of
multivalent antigen in bone
marrow
Peripheral: antigen
presentation by APCs lacking
costimulators; repeated
stimulation by self antigen
Peripheral; antigen recognition
without T cell help or second
signals
Principal mechanism of
tolerance
Central tolerance; deletion
(apoptosis); development of
regulatory T cells
Central tolerance: deletion
(apoptosis); receptor editing
Peripheral tolerance: anergy,
apoptosis; suppression
Peripheral tolerance: block in
signal transduction (anergy);
failure to enter lymphoid
follicles; apoptosis
Immune response to foreign antigens are self-limited and wane as the antigens are eliminated,
returning the immune system to its basal resting state.

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CHAPTER 15
AUTOIMMUNITY: GENERAL PRINCIPLES
Important general concepts
1. Failure or breakdown of mechanism responsible for maintain self tolerance in B-cell, T cell or
both
2. Genetic susceptibility & environmental triggers (infection)
3. May be systemic or organ specific
Introduction
Two categories
A. Normal immune response to abnormal antigen
B. Abnormal immune response to normal antigen (Most significant form perspective of clinical
disease)
Cause
1. Failure of tolerance mechanism
2. Environment & genes
3. Defect in regulation of B or T cell
Normal immune response
1. Antigens hidden (sequestered ag) in cells or tissue (sperm, cornea)
(Some autoimmune response have physiological functions)
e.g. RBCs must be removed from the blood once they reach the end of their life span. This
process is accomplished by autoantibodies. As red cell ages, an anion transport protein called
CD233 is cleaved and a new epitope is exposed. This new epitope is recognized by IgG
autoantibodies. These autoantibodies thus bind to aged red cells and trigger their phagocytosis by
macrophages in the spleen.
Many autoantigens are found in places where they never encounter circulating lymphocytes e.g.
in testes new antigens appear at puberty – long after the T cell system has developed and become
tolerant to autoantigens. Injury to the testes may permit proteins released by damaged tissues to
reach the blood stream, encounter antigen-sensitive cells, and stimulate autoimmunity. Hidden
antigens may also be found inside cells. For example, after a heart attack, autoantibodies may be
produced against the mitochondria of cardiac muscle cells. In chronic hepatitis in dogs, animals

97
develop antibodies to liver membrane proteins. In disease such as trypanosomiasis or
tuberculosis in which widespread tissue damage occurs, autoantibodies to many different tissue
antigens may be detected in serum.
Antigens Generated by Molecular Changes
The production of some autoantibodies may be triggered by the development of completely new
epitopes on normal proteins. E.g. RFs (Rheumatoid factors) auto antibodies against other Igs.
Immunoconglutinin (IKs after the German spelling).
RFs are autoantibodies directed against other Igs. When an antibody binds to an antigen, the
antibody conformation is changed in such a way that new epitopes are exposed on its Fc region.
These new epitopes may stimulate RF formation.
Iks are autoantibodies directed against the complement components C2, C4 and specially C3.
The epitopes that stimulate IK formation are found on sites that are exposed when complement
is activated.
(Molecular Mimicry)
Autoimmunity may result from molecular mimicry, a term used to describe the sharing of
epitopes between an infectious agent or parasite and an autoantigen.
e.g. Trypanosoma cruzi contains antigen that cross react with mammalian neurons & cardiac
muscle. Cell wall M protein of group A streptococci share antigenic properties with cardiac
myosin & glomerular basement membrane. EB (Epstein Barr) virus DNA polymerase cross
reacts myelin basic protein. Polio virus capsid protein VP2 that cross reacts with the acetyl
choline receptor and may induce myasthenia gravis.
(Alternation in antigen processing)
In some case autoimmunity seems to result from an normal immune response against an
exogenous antigen that subsequently spreads to recognize self antigens. Epitope spreading has
been demonstrated in many autoimmune disease such as thyrotoxicosis and diabetes and may
account for the difficulties encountered in controlling these diseases.
Abnormal immune response
(Failure of regulatory control)
Although autoimmunity may be triggered by response to hidden epitopes, a sustained response is
necessary for disease to develop. This may result from a failure of normal control mechanism of
immune system. It is common to find autoimmune diseases associated with lymphoid tumors.

98
e.g. myasthenia gravis is associated with thymic carcinoma. In human there is a four fold
increase in the incidence of rheumatoid arthritis in patients with malignant lymphoid tumors.
Virus Induced Autoimmunity
Many autoimmune diseases appear to be triggered by virus infection. For example, mice infected
with certain reo virus develop an autoimmune poly endocrine disease characterized by diabetes
mellitus and retarded growth. For example SLE of dogs and human has been associated with
type C retro virus or paramyxo virus.
Microchimerism
During pregnancy mother & their fetus may exchange cells. As a result, fetal cells can persist in
a mother’s body for many years after pregnancy and vice versa. This will lead to cause of
autoimmune disease. E.g. In many woman with the autoimmune disease scleroderma, it is
possible to find fetal T, B and NK cells as well as fetal monocytes in their blood stream.

99
CHAPTER 16
ENDOCRINE (ORGAN SPECIFIC AUTOIMMUNE DISEASES)
1. Lymphyocytic thyroditis : It results from the production of auto antibodies against
thyroglobulin. Affected thyroids are infiltrated with plasma cells and lymphocytes, and
germinal centre formation may occur. The infiltrating cells probably cause epithelial cell
destruction through antibody dependent cell mediated cytotoxicity (ADCC) and T cell
cytotoxicity.
2. Hyperthyroidism: It is a disease of old cats. Autoantibodies to thyroid peroxidase have
been demonstrated in almost one third of cases.
3. Lymphocytic parathyroditis: Dogs and cats develop autoimmune hypoparathyroidism.
Auto antibodies against parath hormone is produced. Animal suffer from seizure and
profoundly hypocalcemic.
4. Insulin dependent diabetes mellitus (IDDM): In humans, insulin-dependent diabetes
mellitus (IDDM) is an autoimmne disease mediated by autoantibodies against an islet cell
enzyme called glutamic acid decarboxylase. The canine disease is associated with
pancreatic islets atrophy and a loss of B cells.
5. Autoimmune adrenalitis : Dogs may suffer from lymphocyte mediated destruction of
adrenal cortex. Affected animals present with depression, weak pulse, bradycardia,
abdominal pain, vomiting, diarrhea, dehydration, hypothermia.
Auto immune neurological disease
1. Equine polyneuritis: It is uncommon disease of horse affecting the sacral and coccygeal
nerves (neuritis of cauda equina). Affected horses show hyperesthesia followed by
progressive paralysis of the tail, rectum and bladder.
2. Canine poly neuritis: Canine polyneuritis or coonhound paralysis affects dogs following
a bite or scratch from raccoon. It presents as an ascending symmetrical flaccid paralysis
with mild sensory impairment.
3. Steroid responsive meningitis-arteritis: Corticosteroid-responsive meningitis is
characterized by sterile inflammation of the meningeal arteries and meningitis.

100
AUTOIMMUNE EYE DISEASE
1. Equine Recurrent Uveitis : The most common cause of blindness in horses is equine
recurrent uveitis (for periodic ophthalmia). Horse have recurrent attacks of uveitis,
retinitis and vasculitis.
2. UVEO dermatological syndrome: It is a sporadic disease of dogs. Affected dogs
exhibit severe eye disease (uveitis) and skin depigmentation with whitening of hair
(poliosis) and skin (vitiligo).
AUTO IMMUNE REPRODUCTIVE DISEASE
If the testes are damaged so that hidden antigens are released, then an autoimmune response
may exacerbate the orchitis.
AUTOIMMUNE SKIN DISEASE
1. The pemphigus comlex : The pemphigus complex consists of four skin diseases that
have been described in human, dogs, horses and cats. The most severe is pemphigus
vulgaris. In this disease, bullae develop in the skin around the muco-cutaneous junctions,
especially nose, lips, eyes, preopuce, anus, tongue and the inner surface of the ear.
2. Skin basement membrane disease: A second set of blistering disease is associated with
the development of autoantibodies against components of the skin basement membrane.
e.g. Bullous pemphigoid
3. Alopecia areata (patches like): Alopecia areata is an autoimmune disease directed
against cells in hair follicle. It is characterized by the development of multiple round
spots of hair loss in absence of obvious inflammation.
4. Relapsing polychondritis: A disease involving autoimmunity against type II cartilage
has been described in humans and in cats. The animals present with bilateral curling of
ears and ocular changes. The cartilage is infiltrated with plasma cells and lymphocytes.
AUTO IMMUNE NEPHRITIS
Horses may develop autoantibodies to glomerular basement membrane which may provoke
glomerulonephritis and renal failure.
AUTOIMMUNE HEMOLYTIC ANEMIA
Auto antibodies to RBC antigens provoke their destruction and cause auto immune hemolytic
anemia (AIHA). These hemolytic anemias are well recognized in human and dogs.

101
AUTO IMMUNE THROMBOCYTOPENIA
Auto immune thrombocytopenia (AITP) due to production of auto antibodies to platelets has
been reported in horses, dogs and rarely cats. The clinical sign is excessive bleeding.
AUTO IMMUNE MUSCLE DISEASE
Myasthenia gravis : It is a disease in human, dog & cats of skeletal muscle. It is
characterized by abnormal fatigue and weakness after relatively mild exercise. It results from
a failure of transmission of nerve impulse across the motor endplate of striated muscle as a
result of a deficiency of acetylcholine receptors.

102
CHAPTER 17
SYSTEMIC IMMUNOLOGICAL DISEASES
(Systemic lupus erythematosus, SLE)
SLE is a complex disease of human, horse, dog, cat and other primates.
Pathogenesis: Its development is affected by environmental factors, including infectious
agents, drugs & foods along with many different genes. Patients develop a variety of auto
antibodies (antinuclear), changes in T cell function, defective phagocytosis, and oncogenic
expression. One key defects in lupus patients appears to be the impaired clearance of
apoptopic cells.
SJORGEN’S SYNDROME
The triad of keratoconjunctivitis sicca, xerostomia (mouth dryness), rheumatoid factor (RF)
constitute an autoimmune syndrome called Sjorgen’s syndrome. In this syndrome,
autoimmune attack on salivary & lacrimal glands leads to conjunctival dryness
(keratoconjunctivitis sicca) and mouth dryness (xerostomia). Affected animals subsequently
develop gingivitis, dental caries and excessive thirst. Sjogren’s syndrome is often associated
with rheumatoid arthritis, systemic lupus, polymyositis and autoimmune thyroiditis.
AUTOIMMUNE POLY ARTHRITIS
Deposition of immunoglobulins or immune complexes within joints.
Erosive polyarthritis: The most important immune-mediated erosive polyarthritis in humans
is rheumatoid arthritis (RA). RA is a common, crippling disease affecting about 1% of the
human population.
Pathogenesis: RA is a chronic inflammatory disease. It commences as a synovitis with
lymphocytes in the synovia & neutrophils in synovial fluid. As the inflammation continues,
the synovia swell & proliferate. Outgrowths of the proliferating synovial eventually extends
into joint cavity, where they are called pannus. Pannus consists of fibrous vascular tissue that
invades joint cavity that release proteases & erode articular cartilage.
Diagnostic criteria for canine rheumatoid arthritis
Any four of the following sign must be present
1. Morning stiffness for at least 1 hour for at least 6 weeks

103
2. Arthritis
3. Joints, swelling, exudation
4. Arthritis affecting hand joints longer than 6 weeks
5. Symmetrical arthiritis
6. Rheumatoid nodules
7. RF (Rheumatoid factors)
8. Radiographic change
Non erosive polyarthritis
Joint cartilage is not eroded, inflammatory lesions confined to the joint capsule & synovial
joint.Equine polyarthritis, canine poly arthritis (associated with SLE), lupus poly arthritis,
idiopathic poly arthritis, poly arthritis myositis
Immune vasculitis
Focal necrosis of media. Canine juvenile polyarteritis, polyarteritis nodosa, leukoclastic
vasculitis, necrosis of media, type III hypersensitivity.

104
CHAPTER 18
IMMUNE DEFICIENCY DISEASE
Primary immune deficiencies
Inherited defects in innate immunity
Inherited deficiencies in innate include defects in the various stages of phagocytosi, as well
as complement deficiencies.
(Chediak- Higashi syndrome)
Inherited disease of cattle, Persian cat, white tiger. It is an autosomal recessive disease
resulting from a mutation in a gene (LYST) that regulated lysosomal trafficking within cells.
The defect produces abnormally large granules in neutrophils, monocyte and eosionophil.
The enlarged neutrophil granules result from the fusion of primary and secondary granules.
These leukocytes have defective chemotactic responsiveness, reduced motility and reduced
intracellular killing as a result of defective granule fusion and a deficiency of elastase.
(Pelger-Huet anomaly)
It is an inherited disorder characterized by a failure of granulocyte nuclei to segment into
lobes. As a result their nuclei remain round in shape.
Canine leukocyte adhesion deficiency
It results from a defect in the integrins. In this disease neutrophils cannot respond to a
chemotaxis. Affected dogs have recurrent infections, while at the same time, they have large
number of neutrophils in their blood.
Bovine leukocyte adhesion deficiency
An integrin deficiency occurs in Holstein calves. It is an autosomal recessive trait
characterized clinically by recurrent bacterial infections, anorexia, oral ulcerations, gingivitis
and a persisten extreme neutrophilia.
Canine cyclic neutropenia (Gray Collie syndrome)
It is an autosomal recessive disease of collies. Affected digs have dilution of skin
pigmentation, eye lesions and regular cyclic fluctuations in leukocyte number. The loss of
neutrophils occurs about every 11-12 days, lasts for 3 days. It is followed by normal or
elevated neutrophil counts for about 7 days.

105
IMMUNO DEFECIENCIES OF HORSES
(Severe combined immune deficiency)
The most important congenital equine immune deficiency is the severe combined immune
deficiency syndrome (SCID). Affected foals fail to produce functional T or B cells and have
very few circulating lymphocytes.
Immunodeficiencies of cattle
SCI (severe combined immuno deficiencies) → selective IgG2 deficiency, hereditary
parkeratosis
Immunodeficiencies of dogs
SCI → Ig deficiency, T cell deficiency, uncharacterized immune deficiency
(Immunodeficiencies of cats)
Hypotrichosis with thymic aplasia
(Immuno deficiencies of chicken)

106
CHAPTER 19
SECONDARY IMMUNE DEFICIENCIES
Virus induced immune suppression
Secondary lymphoid tissue
Viruses that destroy lymphoid tissue. HIV, measles, SIV (Simian influenza virus), FIV
(Feline immuno deficiency virus), feline leukemia virus, distemper, African swine fever,
bovine viral diarrhea, pes-des-petits ruminants (PPR), infectious bursal disease, Newcastle
disease.
Virus that stimulate lymphoid tissue activity to an unusual extent
Maedi-virus, Aleutian disease virus, malignant catarrhal virus
Viruses that cause lymphoid neoplasia
Marek’s disease, feline leukemia virus, bovine leukemia virus, mouse leukemia virus,
human T- cell leukemia virus -1
Other cause of secondary immunodeficiency
Microbial & parasite infections
Toxoplasma, trypanosomes, heminths (trichinella spiralis), demodex, & bacteria (Mannhemia
hemolytica, actinobacilli & some streptococci)
Toxin induced
Polychlorinated biphelnyls, poly brominated biphenyl dieldrin, iodine, lead, cadmium,
methyl mercury & DDT.
Malnutrition
Severe nutritional deficiencies (A, B, D, Zn, taurine deficiency in cats)
Exercise : strenuous
Post traumatic immuno deficiencies
Age: old age
Other secondary Immune deficiencies
Protein loss in kidney disease, parasitized tumor bearing, severe burn, trauma

107
CHAPTER 20
TRANSPLANTATIONAL IMMUNOLOGY
Transplantation: Process of taking cells, tissue or organs called a graft from one individual &
replacing than into different individual. The individual who provides the graft is called
donor, and the individual who receives the graft is called recipient.
Orthotopic transplantation: If the graft is placed into its normal anatomic location, the
procedure is called orthotopic transplantation.
Heterotopic transplantation: If the graft is placed in different site, the procedure is called
heterotropic transplantation.
Transfusion: Transfer of circulating blood cells or plasma from one to other
Major limiting factor of successful transplantation is immune response of recipient to donor
tissue.
Failure of skin grafting caused by inflammatory reaction rejection
Autologus graft: A graft transplanted from one individual to the same individual is called an
autologus autograft
Syngeneic graft: A graft transplanted between two genetically identical or syngeneic
individuals is called a syngeneic graft.
Allogenic graft: (allograft): A graft transplanted between two genetically different
individuals of same species is called an allogenic graft or allograft.
Xenogenic graft or xenograft : A graft transplanted between individuals of different species
is called xenogeneic graft or xenograft.
Alloantigens, xenoantigens: The molecules that are recognized as foreign on allografts are
called alloantigens, and those on xenograft are called xenoantigens. Lymphocytes &
antibodies that react are called alloreactive or xenoreactive
Importance:
1. Immunological rejection
2. Useful mode for studying mechanism of lymphocyte activation
Immune response to allograft
Alloantigens → cells mediated & humoral immune response

108
Recognition of alloantigens
1. Recognition of transplanted cells as self or foreign is determined by polymorphic genes
that are inherited from both parents & are expressed co-dominantly. MHC complex is
locus for Polymorphic gene.
2. MHC molecules are responsible for almost all strong (rapid) rejection reactions.
Allogenic MHC molecules are presented in 2 ways
1. Direct presentation
2. Indirect presentation
Direct presentation: Donor APC (Antigen Presenting Cell) in the graft present MHC to
recipient to T-cells.
Indirect presentation: processing of donor MHC molecules by recipient APCs &
presentation along with self MHC molecule.
Allo antigens other than MHC molecule in the graft is also presented by indirect pathway.
MHC II & MHC I presentation: Cross presentation
Minor Histocompatibility antigens: Polymorphic antigens other than MHC molecules
which induce slow or weak rejection.
Activation of alloreactive lymphocytes
Alloreactive CD4+ helper T cells differentiate into cytokine producing effector cells that
damage grafts by reaction that resembles delayed type hypersensitivity.
Alloreactive CD8 + T cells → direct presentation → killing acute rejection CD8+ T cells
Chronic rejection CD4+ T cells
B-7 is also important
In contrast to T- cell reactivity → less is known about the mechanism of alloantibody
production.
Effector mechanism of graft rejection
Graft rejection, histopathological feature, time course of rejection → hyperacute, acute &
chronic.
Hyperacute
Thrombotic occlusion of the graft vasculature that begins within minutes to hrs → mediated
by pre-existing antibodies against donor endothelial antigen.

109
Acute
Vascular & parenchymal injury → T cells & abs after week of transplantation
Chronic rejection & graft vasculopathy
Vascularized grafts survive for more than 6 months
Chronic DTH (delayed type hypersensitivity) injury to vessel wall → intimal smooth muscle
cell proliferation → luminal occlusion
Prevention & treatment of allograft rejection
Immuno suppression to prevent or treat allografts rejection (drugs)
1. Cyclosporin & FK-506: Block T cell cytokine production
2. Azathioprine: blocks proliferation of lymphocyte precursor
3. Mycophenolate mofetil: block lymphocyte proliferation
4. Rapamycin: Blocks lymphocyte proliferation
5. Corticosteroid: reduce inflammation by inhibiting macrophage cytokine secretion
6. Anti CD3 mab: depletion of T Cells
7. Anti IL-2 receptor ab: inhibit T cell proliferation
8. CTLA-4 →Ig → block B7 co-stimulator
9. Anti CD40L → blocks cd40: macrophage & endothelial activation
(Method to reduce immunogenicity of allograft)
ABO blood group antigen → cross matching, tissue typing (HLA matching)
Methods to induce donor specific tolerance
Blood transfusion
Kidney transplants
Bone marrow transplants from donor
Inducing regulatory T –cells for graft alloantigens
Graft versus Host disease (GVHD) is principal limitation to the success of bone marrow
transplantations

110
CHAPTER 21
TUMOR IMMUNOLOGY
Certain chemicals, virus, mutation, cells may occasionally break free of regulation of normal cell
division and give rise to tumor. If these cells remain clustered together at a single site, the tumor
is said to be benign. If tumors arise in the distant site are called malignant. Malignant tumors are
subdivided according to their tissue of origin. Tumors arising from epithelial cells are called
carcinoma, those arising from mesenchymal cells such as muscle, lymphoid or connective tissue
are called sarcoma. A leukemia is a tumor derived from hematopoietic cells.
Tumors as allograft
Immunosuppression →increase incidence of tumors
For example, patients with AIDS may develop Kaposi’s sarcoma. From this suggestion the
immune surveillance theory emerged. This theory held that the body constantly produces
neoplastic cells but that in a healthy individual the immune system rapidly recognizes and
eliminates these cells through cell mediated mechanism.
Tumor associated antigens
Cancer may be diagnosed on production of new proteins by tumor cells. These may simply be
normal proteins produced in excessive amounts.
Production of PSA (prostate specific antigen) is found in prostate carcinoma
Some tumor may express the products of developmental genes that are turned off in adult cells
and are normally only expressed early in an individual’s development. These proteins are called
oncofetal antigen. e.g. carcinoembryonic ag, CEA is found in only in fetal intestine. Αlpha
fetoprotein produced by hepatoma cell is also an example of oncofetal antigen since it is
normally found only in the fetal liver.
Tumor cells rarely develop new antigens. Some of the tumor antigens are recognized because
they are abnormally glycosylated. Melanoma express characteristic gangliosides and many
carcinomas express mucin.
Virus induced tumor → new antigen, Marek’s disease tumor cells (MATSA- Marek’s disease
associated tumor surface antigen)

111
Chemical → different mutation → different tumors in different animals
Marek’s disease associated tumor surface antigen
Immunity to tumors
In tumor abnormal molecules are not appropriately presented to the cells of the immune system,
especially cytotoxic T cells. In tumor most important cell is NK cell.
NK cells
About 15% of mammalian blood lymphocytes are neither T or B cells but belong to a third
population of lymphocytes called NK cells. It kill many types of tumor cells, especially cells that
have reduced class I MHC expression but do express ligands for NK cells activating receptors.
NK cells can be activated almost immediately by IFN from virus infected cells and by IL-12
from macrophage. NK cells are concentrated mainly in the secondary lymphoid organ, a few are
found in the bone marrow and none is found in thymus.
Surface marker: NK cells express a characteristic major surface antigen called CD56.
Target cell recognition
Triggering of NK cell cytotoxicity results from a change in the balance between activating &
inhibitory signals. When NK cells encounter normally body cells, inhibitory signals predominate.
MHC class I molecules on the surface of healthy normal cells provide inhibitory signals
sufficient to block NK cell killing.
A second mechanism that triggers NK cell cytotoxicity involves the recognition of proteins on
the target cell surface called MICA (major histocompatibility complex, class I chain related A)
and MICB. They are not expressed on normal healthy cells. Natural killer cells have a receptor
called NKG2D that binds to MICA and MICB.
NK cells also recognize target cells by a third, antibody-dependent process using CD16. CD16 is
an Fc receptor expressed on NK cells, granulocytes and macrophages. NK cell may detach from
an antibody-coated target cell after delivering the lethal hit.
Effector mechanism
Like T cells through perforin & granzymes as well as through death receptor pathways involving
CD95L, TNF α, TNFβ. Perforin and granzymes are constitutively expressed in NK cells. It
produces characteristic small (5-7 nm) lesion in target cell surface. Presumably granzymes are
injected into the target cells in association with perforin channels. CD178 (CD95L) on NK cells
can induce apoptosis in target cells by binding to CD95 on the target cell surface.

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