immunology basics

1. COURSE CODE:BCM 127
COURSE TITLE: BASIC AND CELLULAR IMMUNOLOGY
1

2. COURSE OUTLINE
 Course instructor: Dr. E. Mibei
Purpose of the Course
 The course is meant to introduce the students to basic and
contemporary concepts in immunology.
Expected Learning Outcomes
 By the end of this course, the students should be able to:
1. Describe the historical background, definitions and terms
used in immunology.
2. Classify and characterize the features of functional
immunity.
3. Describe the cells and organs of the immune system,
histocompatibility complex and the ontogeny of T and B
lymphocytes, antigen presentation and cytokines.
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3.  Course delivery methods
 Lectures, Practicals, Tutorials, Group discussions
relevant hand outs.
 Course assessment
 Continuous assessment tests (CATs) 20%
 Assignments/Practicals 10%
 Final written examination 70%
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4. Course content
 Introduction to immunology; Definitions and
terminology used in immunology; Historical background
of immunology; Resistance and immunity;
 Classification and characteristic features of functional
immunity: Innate and Acquired immunity; primary and
secondary immune responses; Humoral immunity; Cell-
mediated immunity;
 Cells and Organs of the immune system: Cells;
Granulocytes; lymphocytes; Ontogeny and development
of T and B lymphocytes; Lymphoid organs and tissues;
Primary; secondary organs/tissues
 The Major Histocompatibility Complex: Antigen
processing and presentation; Cytokines.
4

5. Basic Concepts in Immunology
Immunology – study of the immune system
Immunity – immunis – free of, free of the burden of taxes
or military conscription
 Immunity is the ability to resist damage from foreign
substances (microorganisms, harmful chemicals).
 Immunity is categorized as being innate or adaptive.
5

6. Disease – abnormal state in which part or all of the
body is not properly adjusted/not capable of
performing normal functions, deviation from
normal
Pathology – scientific study of disease, etiology
(cause) pathogenesis (development) and effects of
disease
Pathogen – disease causing microorganism
Host – an organism that shelters and support
growth of parasite/pathogens
Infestation – invasion by animal parasites to the
external surface
6

7.  Infection – invasion of the body by disease causing
organisms with or without development of disease
 Resistance – the ability to withstand infestation or
infection by a living organism, insusceptibility. Can be;
 Natural – innate, inborn, is inherited and unchanging
 Acquired – developed subsequent to exposure, is
adaptive in nature
7

8.  Main function of the immune system is to protect the
body from damage caused by microorganisms;
bacteria, fungi, viruses and other parasites
 The defensive function is performed by WBCs and a
number of accessory cells
 Cells distributed but found particularly in lymphoid
organs whish are strategically placed in the body
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9.  The immune system
 Nonspecific or innate defense system
 Cellular
 Humoral
 Specific or acquired immune system
 Cellular
 Humoral
The immune system - classification
9

10.  Innate immunity provides the basic means for the
destruction of foreign organisms. It recognizes and
destroys certain foreign substances, but the response
to them is the same during each encounter. It consists
of mechanical barriers as well as certain cells and
chemical mediators.
10

11.  The main barriers are skin and mucosae. Cells and
chemicals include granulocytes, monocytes,
macrophages, antimicrobial proteins, surface
secretions etc.
 A characteristic response of the innate system is
inflammation.
11

12.  The adaptive immune system consists of cells that
attack particular antigens in a particular way. It
improves and enhances the efficiency of the innate
mechanisms and remembers the infection the next
time it is encountered.
 Specificity (the ability to distinguish pathogens)
and memory (the ability to respond more rapidly
to a previously encountered pathogen) are
characteristics of adaptive immunity
12

13. 13

14. Skin and mucous membranes
Mechanical and chemical factors
 Skin helps repel pathogens in many ways. It’s
highly keratinized, which provides a physical
barrier to pathogens. The acidity of sweat can kill
some pathogens. Sebum is bactericidal.
 Mucous membranes are another major barrier
to pathogens. They line the digestive, respiratory,
urinary, and reproductive tracts - all of which are
potential entrance points for pathogens. They are
often covered in sticky, pathogen-trapping mucus.
14

15.  Respiratory mucosa is also ciliated. Cilia sweep
bacteria-laden mucus upward to the pharynx
where it can be swallowed. Coughing and sneezing
also assist in expulsion.
 A variety of body fluids also provide innate
defense. Tears, saliva, and urine wash away
microorganisms. Saliva, intestinal fluid, and tears
contain lysozyme, an enzyme that destroys
bacteria. The acidity of certain mucosal secretions
(gastric and vaginal) can impair pathogens.
15

16.  First line of defense system
 It distinguishes self from non-self but does not distinguish one type of
pathogen from another.
 Components:
 skin and mucous membranes
 Biochemical mediators/Surface secretions
 inflammatory response and fever
 phagocytic and non phagocytic leukocytes cells
 Normal Flora
 Complement system
 Other Chemicals; interferon
Nonspecific or innate defense system
16

17.  1. Mechanical factors
 The epithelial surfaces form a physical barrier that is very
impermeable to most infectious agents.
 The skin acts as our first line of defense against invading organisms.
The desquamation of skin epithelium also helps remove bacteria
and other infectious agents that have adhered to the epithelial
surfaces.
 The skin is relatively dry and does not support colonization
 Movement due to cilia or peristalsis helps to keep air passages and
the gastrointestinal tract free from microorganisms.
 The flushing action of tears and saliva helps prevent infection of the
eyes and mouth.
 The trapping effect of mucus that lines the respiratory and
gastrointestinal tract helps protect the lungs and digestive systems
from infection.
Nonspecific defense system
17

18. Skin and mucous membranes - Mechanical and
chemical factors
 Skin helps repel pathogens in many ways. It’s
highly keratinized, which provides a physical
barrier to pathogens. The acidity of sweat can kill
some pathogens. Sebum is bactericidal.
 Mucous membranes are another major barrier
to pathogens. They line the digestive, respiratory,
urinary, and reproductive tracts - all of which are
potential entrance points for pathogens. They are
often covered in sticky, pathogen-trapping mucus.
18

19.  Respiratory mucosa is also ciliated. Cilia sweep
bacteria-laden mucus upward to the pharynx
where it can be swallowed. Coughing and sneezing
also assist in expulsion.
 A variety of body fluids also provide innate
defense. Tears, saliva, and urine wash away
microorganisms. Saliva, intestinal fluid, and tears
contain lysozyme, an enzyme that destroys
bacteria. The acidity of certain mucosal secretions
(gastric and vaginal) can impair pathogens.
19

20.  Fatty acids in sweat inhibit the growth of bacteria.
 Lysozyme and phospholipase found in tears, saliva
and nasal secretions can breakdown the cell wall of
bacteria and destabilize bacterial membranes.
 The low pH of sweat and gastric secretions prevents
growth of bacteria.
 Defensins (low molecular weight proteins) found in
the lung and gastrointestinal tract have
antimicrobial activity.
 Surfactants in the lung act as opsonins (substances
that promote phagocytosis of particles by
phagocytic cells).
 Transferrins in blood
2. Chemical factors
20

21. 1st Line
Defense in
Human

22. 3. Biological factors/Normal Flora
 Normal flora/commensals - growth of disease-causing
organisms is inhibited by the growth of non-pathogenic
bacteria in the skin, gastrointestinal and urogenital tracts.
 This is mainly through antagonism and competitive
exclusion abilities of the normal microbiota
 These bacteria successfully compete with the pathogenic
ones for nutrients and resources or attachment to cell
surfaces and can also
 Secrete harmful/toxic substances targeting the non-
commensals e.g. bacteriocins
 Modify the environment to the detriment of pathogenic
strains e.g. lactose fermenting bacteria in the vagina
 Honey to treat wounds.pdf
22

23.  The anatomical barriers are very effective in preventing
colonization of tissues by microorganisms.
 However, when there is damage to tissues the anatomical
barriers are breached and infection may occur. Once
infectious agents have penetrated tissues, another innate
defense mechanism comes into play, namely acute
inflammation.
 Humoral factors play an important role in inflammation,
which is characterized by edema and the recruitment of
phagocytic cells.
Nonspecific defense system
23

24. 4. Cellular factors
 WBCs and derivatives are the most important cellular
component of the innate immune system.
 WBCs can exit blood vessels (diapedesis), converge
upon areas of infection/damage (positive chemotaxis)
and move over, between, and through other cells.
 Examples: Neutrophils, macrophages, eosinophils, NK
cells
 Mechanism is phagocytosis (enhanced by
opsonization) and inflammation.
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25. Immune Cells
25

26. Second Line of Defense: Formed Elements in Blood
20-25%
3-8%
0.5-1%%
2-4%
60-70%
26

27.  Phagocytes:
 Neutrophils – bacteria
 Eosinophils – enzymes that kills parasites
 Monocytes and Tissue specific macrophages
 Macrophages - "big eaters"
 Non phagocytic leukocytes:
 Basophils – role in allergic response
 Mast cells
 Natural killer cells/Large granular lymphocytes – antiviral and anti-
tumor activity
Nonspecific immune cells
27

28.  Macrophages have important functions in both innate and antigen-
specific immune responses.
 As phagocytic cells, they target the non self in a nonspecific manner,
they help to contain infectious agents until specific immunity can be
marshaled.
 In addition, early in the host response, the macrophage functions as an
accessory cell to ensure amplification of the inflammatory response
and initiation of specific immunity.
 Macrophages are activated by the presence of antigen to engulf and
digest foreign particles.
 Activated macrophages act as antigen presenting cells (APCs) that
break down complex antigens into peptide fragments that can associate
with class I or II Major Histocompatibility Complex (MHC) molecules.
Macrophages can then present these complexes to the helper T cell so
that nonself-self recognition and activation of the immune response
can occur.
Nonspecific immune cells
28

29. Process of Phagocytosis
Phagocytes engulf and kill microorganisms
Steps of phagocytosis:
• Chemotaxis
• Recognition and attachment
• Engulfment and creation of phagosome
• Fusion of phagosome with lysosome
• Destruction and digestion
• Residual body  Exocytosis
29

30. Phagocytosis
Foundation Fig
16.7 30

31. Inhibit adherence: M
protein, capsules
Streptococcus pyogenes, S.
pneumoniae
Kill phagocytes:
Leukocidins
Staphylococcus aureus
Lyse phagocytes:
Membrane attack complex
Listeriamonocytogenes
Escape phagosome Shigella
Prevent phagosome-
lysosome fusion
HIV
Survive in
phagolysosome
Coxiella burnetti
Microbial Evasion of Phagocytosis
31

32.  The complement system is a primary mediator of the
humoral immune response that enables the body to
produce an inflammatory response, lyse foreign cells,
and increase phagocytosis.
 The complement system, like the blood coagulation
system, consists of a group of proteins that normally are
present in the circulation as functionally inactive
precursors. These proteins make up 10% to 15% of the
plasma protein fraction.
 For a complement reaction to occur, the complement
components must be activated in the proper sequence.
 Uncontrolled activation of the complement system is
prevented by inhibitor proteins.
5. The Complement System
32

33. Complement System
Series of  30 plasma (serum) proteins,
activated in a cascade
Three effects of complement system:
1. Enhances inflammatory response, e.g.:
attracts phagocytes
2. Increases phagocytosis through
opsonization or immune adherence
3. Creates Membrane Attack Complexes
(MACs)  Cytolysis
33

34.  The classic pathway of complement activation is initiated
by antibody bound to antigens on the surface of microbes
or through soluble immune complexes.
 The alternate and the lectin pathways do not use
antibodies and are part of the innate immune defenses.
 The alternate pathway of complement activation is
initiated by the interaction with certain polysaccharide
molecules characteristic of bacterial surfaces.
 The lectin-mediated pathway is initiated following the
binding of a mannose-binding protein to mannose-
containing molecules commonly present on the surface
of bacteria and yeast.
 The activation of the three pathways produces similar
effects on C3 and subsequent complement proteins.
Complement activation
34

35. Complement activation
35

36. Opsonins (complement proteins or antibodies) coat
bacteria and promote attachment of micro-organism
to phagocyte  Opsonization
36

37. Classical Pathway
37

38. Alternative Pathway
Does not require a
specific antibody
to get started
38

39. Some Bacteria Evade Complement
 Capsules prevent Complement activation.
 Surface lipid-carbohydrates of some Gram-
negatives prevent MAC formation.
 Enzymatic digestion of C5a by Gram-positives.
39

40. Interferons (IFNs)
 Family of glycoproteins
 Host-cell-specific but not virus-specific
 -IFN and -IFN: Produced by virus infected cells.
Mode of action is to induce uninfected cells to produce
antiviral proteins (AVPs) that inhibit viral replication.
  -IFN: Produced by lymphocytes. Causes neutrophils
and macrophages to phagocytize bacteria. Also
involved in tumor immunology.
 Recombinant interferons have been produced.
However short-acting and many side-effects.
40

41. Interferons (IFNs)
41

42. Summary so far
 The immune system protects the body against
disease. The roles of the immune system include:
1) recognizing the presence of an infection;
2) containing the infection and working to eliminate it;
3) regulating itself so that it does not damage the body
4) remembering pathogens to prevent disease from
recurring.
42

43. DEFINITIONS:
 Antigen (Ag): The immune system identifies specific foreign agents,
contains and attacks them. These foreign agents (anything that is non-
self) are known as antigens. Every cell has antigens; antigens are
markers on the surface of cells that the immune system can recognize.
Self antigens are antigens in individual to which the immune system is
tolerant (the immune system does not try to attack)
 Ag is molecule which elicits a specific immune response when
introduced into an animal. More specifically, antigenic (immunogenic)
substances are:
 Generally large molecules (>10,000 daltons in molecular weight),
 Structurally complex (proteins are usually very antigenic),
 Accessible (the immune system must be able to contact the molecule), and
 Foreign (not recognizable as "self").
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44. Definitions 2
 The antigen is usually of sufficient size to contain a
specific marker that triggers an immune response e.g.
antibody production.
 Small substances that can't be recognized by the
immune system are called haptens; when joined with a
protein the immune system can recognize them.
 Also it is very important to remember that the
immune system can distinguish between self and non-
self.
44

45. Definition 3
 Antibody (Ab): A glycoprotein produced in
response to an antigen that is specific for the
antigen and binds to it via non-covalent
interactions. The term "immunoglobulin" is often
used interchangeably with "antibody". We will use
the term "immunoglobulin" to describe any
antibody, regardless of specificity, and the term
"antibody" to describe an antigen-specific
"immunoglobulin". Immunoglobulins (Igs) come
in different forms (IgA, IgD, IgE, IgG, IgM) that
reflect their structure.
45

46. SELF VERSUS NON-SELF
 Cells have surface antigens that are coded by clusters of genes
called Major Histocompatibility Complex (MHC). Many of these
genes are found on chromosome 6.
 HLA stands for Human Leukocyte Antigens because they were
first detected on the leukocyte on the human and also inherited
and part of genetic makeup.
 They are the same thing as the MHC, just a different name.
Another way to kook at it is the MHC refer to genes that code for
the antigens which are referred to as the HLA.
 We have so many different tissue antigens that its virtually
impossible for two people to be identical unless they are
identical twins
 MHC roles
 determines resistance and susceptibility to disease
 Tissue transplant rejection or acceptance
 Sexual mate selection
46

47. Ags vs Abs vs MHC
 Every cell has antigens; antigens are markers on the
surface of cells that the immune system can recognize.
 Self antigens are antigens in individual to which the
immune system is tolerant (the immune system does
not try to attack).
 So there is self tolerance which is induced during
lymphocyte development
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48. Part TWO
48

49. Immune Response
 Host defense is present in many forms. Overall,
the Immune Response (IR) can be divided into two
major classifications; humoral and cell-
mediated.
 Humoral immunity is immunity in fluids and is
antibody mediated (cell-free bodily fluid or
serum)
 CMI is mediated by immune cells and is effected
through cytotoxicity
 While these responses are not mutually exclusive,
they provide distinctly different avenues for
dealing with pathogenic organisms or altered host
cells.
49

50. Humoral Immunity
 The humoral immune system, also known as antibody-mediated
response, protects against micro-organisms present in the fluids of the
body.
 The main component of the humoral immune response is B cells.
 Humoral immunity is important in eliminating bacteria, neutralizing
bacterial toxins, and preventing viral reinfection and hypersensitivity
 The production of antibody involves three distinct phases:
 Induction phase: Ag reacts with specific T and B cells
 Expansion and Differentiation phase: Induced lymphocyte clones
proliferate and mature to a functional stage (i.e. Ag receptor cells mature to
Ag effector cells)
 Effector phase: Abs or T cells exert biological effects either:
 Independently or
 Through the action of macrophages, complement, other non-specific agents
50

51. Cell Mediated Immunity
 CMI is an immune response that does not involve
antibodies but rather involves the activation of
phagocytes, natural killer cells (NK), antigen-
specific cytotoxic T-lymphocytes, and the release
of various cytokines in response to an antigen
 Historically the protective function of
immunization was associated with cells. CD4 cells
or helper T cells provide protection against
different pathogens.
 Cytotoxic T cells cause death by apoptosis without
using cytokines, therefore in cell mediated
immunity cytokines are not always present.
51

52.  The cell-mediated immune system protects against micro-
organisms that have infected cells.
 Infected cells will have identifying molecules on their
surface that trigger the cell-mediated immune system to
respond.
 The cell-mediated immune system also protects the body
by fighting cancerous cells. The main component of the
cell-mediated immune response is T cells.
 Cell-mediated immunity is directed primarily at microbes
that survive in phagocytes and microbes that infect non-
phagocytic cells.
 It is most effective in removing virus-infected cells, but also
participates in defending against fungi, protozoans,
cancers, and intracellular bacteria. It also plays a major role
in transplant rejection.
52

53.  Cellular immunity protects the body by:
 activating antigen-specific cytotoxic T-lymphocytes that
are able to induce apoptosis in body cells displaying
epitopes of foreign antigen on their surface, such as
virus-infected cells, cells with intracellular bacteria, and
cancer cells displaying tumor antigens;
 activating macrophages and natural killer cells, enabling
them to destroy pathogens; and
 stimulating cells to secrete a variety of cytokines that
influence the function of other cells involved in adaptive
immune responses and innate immune responses.
53

54. Primary vs. Secondary Immune Response
 The primary immune response occurs the first time
that the immune system comes in contact with the
antigen.
 During this time the immune system has to learn to
recognize antigen and how to make antibody against
it and eventually gain immunological memory.
 This primary response takes time (about two weeks)
and during this time the person experiences signs of
illness.
 IgM antibodies are the hallmark of a new infection
because they are the first antibodies made when a
person is exposed to an antigen for the first time.
 After the body learns to make IgM antibodies, it will
start making IgG antibodies to the antigen.
54

55.  The secondary immune response occurs the second
time (3rd, 4th, etc.) the person is exposed to the same
antigen.
 At this point immunological memory has been
established and the immune system can mount an IR
immediately e.g. start making antibodies immediately.
 The antigen is usually killed within minutes and the
person is not aware that he/she was attacked – no
symptoms develop
 The antibodies in this response are IgG and IgA or (in
the case of allergy IgE).
55

56. Classification of acquired
Immunity
56
Acquired Immunity
Passive acquired
IR response raised
elsewhere (outside)
Active Acquired
IR raised from within
the host
Natural
Placental
transfer of
abs
Transfer of
abs through
breastfeeding
Artificial
Administration of
anti tetanus or anti
gas gangrene or
anti snake venom
antibodies. Also
transfer of
activated T cells to
treat TB
Artificial
Administration of a
vaccine preparation
(during
immunization) to
mimic natural
infection
Natural
Recovery
from clinical
or subclinical
infection

57. Part THREE
57

58. CELLS OF THE IMMUNE SYSTEM
 All blood cells and immune cells arise from the pluri-potent stem cell
(which is a progenitor cell arising from early developmental stages)
through the process of haematopoiesis
 Haematopoiesis takes place in the Foetal liver, Foetal spleen and later
in the bone marrow of the long bones.
 The spleen and the liver can supplement the BM in cases of chronic
infections or in case of extensive blood loss
 Cells of the immune system are found in the bone marrow, lymph
nodes, spleen, thymus and tonsils.
 There are many different kinds of cells that work as part of the immune
system.
 Most immune system cells are white blood cells or leukocytes. The five
types of white blood cells are neutrophils, eosinophils, basophils,
monocytes and lymphocytes.
58

59.  Under the influence of specific messenger molecules –
cytokines, and as dictated by the need, the pluripotent
stem cell can develop to:
 Lymphoid stem cell or Myeloid stem cell
 LSC depending on the prevailing need, can develop to T
or B cells or NK cells
 MSC develops to platelets, RBCs or the granulocyte-
monocyte lineage
 NB. The development is dependent on specific
cytokine growth factors
59

60. 60

61.  Immune responsive cells can be divided into five
groups based on:
 the presence of specific surface components and
 function: B-cells (B lymphocytes), T-cells (T
lymphocytes), Accessory cells (Macrophages and other
antigen-presenting cells), Killer cells (NK and K cells),
and Mast cells.
 NB: any specific IR can only be mounted in presence of
lymphocytes hence all verts have heterogenous
lymphocytes
61

62. 62
Cell group Surface components Function
B-lymphocytes •Surface immunoglobulin (Ag
recognition)
•Immunoglobulin Fc receptor
•Class II Major Histocompatability
Complex (MHC) molecule (Ag
presentation)
•Direct antigen recognition
•Differentiation into antibody-producing
plasma cells
•Antigen presentation within Class II MHC
T-lymphocytes •CD3 molecule
•T-cell receptor (TCR, Ag recognition)
•Involved in both humoral and cell-
mediated responses
•Helper T-cells (TH) •CD4 molecule
•Recognizes antigen presented within
Class II MHC
•Promotes differentiation of B-cells and
cytotoxic T-cells
•Activates macrophages
•Suppressor T-cells
(TS)
•CD8 molecule •Downregulates the activities of other
cells
•Cytotoxic T-cells
(CTL)
•CD8 molecule •Recognizes antigen presented within
Class I MHC
•Kills cells expressing appropriate antigen

63. 63
Accessory cells Surface components Function
•Macrophages
•Variable
•Immunoglobulin Fc
receptor
•Complement
component C3b receptor
•Class II MHC molecule
•Phagocytosis and cell killing
•Bind Fc portion of immunoglobulin (enhances
phagocytosis)
•Bind complement component C3b (enhances
phagocytosis)
•Antigen presentation (internalized) within Class II
MHC
•Secrete IL-1 (macrokine) promoting T-cell
differentiation and proliferation
•Can be "activated" by T-cell lymphokines
•Dendritic cells •Class II MHC molecule •Antigen presentation within Class II MHC
•Polymorphonuclear
cells (PMNs)
•Immunoglobulin Fc
receptor
•Complement
component C3b receptor
•Bind Fc portion of immunoglobulin (enhances
phagocytosis)
•Bind complement component C3b (enhances
phagocytosis)

64. 64
Cell group Surface components Function
•Killer cells •Variable •Direct cell killing
•NK cells •Unknown •Kills variety of target cells (e.g. tumor cells,
virus-infected cells, transplanted cells)
•K cells
•Mast cells
•Immunoglobulin Fc
receptor
High affinity IgE Fc
receptors
•Bind Fc portion of immunoglobulin
•Kills antibody-coated target cells (antibody-
dependent cell-mediated cytotoxicity, ADCC)
•Bind IgE and initiate allergic responses by
release of histamine. Found in tissues and
function as basophils

65.  Eosinophils – granules with eosinophilic mediators
that are toxic to many organisms and tissues as in
asthma and allergic reactions
 Are phagocytic and useful in parasitic infections like
helminths and large parasites
 Basophils – very few in circulation and known to
function in type I hypersensitivity
 Have high affirnity Fc receptors for IgE (FcƐR)
 Function as Mast cells but in blood not tissues
 Crosslinking IgE on the surface causes basophils to
degranulate and release potent chemical mediators
including heparin, histamine, bradukinins etc.
65

66. Lymphocytes
 Derived from stem cells in the bone marrow.
 Stem cells produce the specialized blood cells.
 Replace themselves by cell division so the stem cell
population is not depleted.
 Lymphocytes seed the thymus, spleen, and lymph
nodes.
66

67. Lymphocytes
 Lymphocytes that seed the thymus become T
lymphocytes (T cells).
 Have surface characteristics and immunological
function that differ from other lymphocytes.
 Do not secrete antibodies.
 Must come in close or direct contact to destroy
them.
 T cells are 65 – 85% of the lymphocytes in blood
and most in the germinal centers of lymph nodes
and spleen.
67

68. Lymphocytes
 Most of the lymphocytes that are not T cells are B
lymphocytes (B cells).
 Processed in the bone marrow.
 Function in specific immunity.
 B cells combat bacterial infections as well as some
viral infections by secreting antibodies into the
blood and lymph.
 Provide humoral immunity (blood and lymph are
body fluids (humors).
68

69. B Lymphocytes
 Secrete antibodies that bind to antigens.
 Stimulate production of memory cells:
 Important in active immunity.
 Others are transformed into plasma cells:
 Produce 2000 antibody proteins/sec when exposed to
antigen.
 These antigens may be isolated molecules or may be
molecules at the surface of an invading foreign cell.
69

70. 70

71. Antibodies
 Antibody proteins are also known as
immunoglobulins.
 Found in the gamma globulin class of plasma proteins.
 Different antibodies have different structure, as the
antibodies have specific actions.
71

72. 72
Antibodies
Immunoglobulin Functions
lgG Main form of antibodies in circulation:
production increased after immunization;
secreted during secondary response
lgA Main antibody type in external secretions, such
as saliva and mother’s milk
lgE Responsible for allergic symptoms in immediate
hypersensitivity reactions
lgM Function as antigen receptors on lymphocyte
surface prior to immunization; secreted during
primary response
lgD Function as antigen receptors on lymphocyte
surface prior to immunization; other functions
unknown

73. Antibody Structure
 100 million trillion
antibody molecules that
contain 4 polypeptide
chains.
 Fab regions are variable,
provide a specific
bonding site for antigen.
 B lymphocytes have
antibodies that serve as
receptors for antigens
 Provides active
immunity.
73

74. Lymphocyte Clones
 Clonal selection hypothesis (Jerne and Burnet): The
clonal selection hypothesis attempts to explain the findings
that the immune system is specific and has memory and
suggest the following:
 Animals contain numerous cells called lymphocytes,
 Each lymphocyte is responsive to a particular antigen by
virtue of specific surface receptor molecules,
 Upon contacting its appropriate antigen, the
lymphocyte is stimulated to proliferate (clonal
expansion) and differentiate,
 The expanded clone is responsible for the secondary
response (more cells to respond) while the differentiated
("effector") cells secrete antibody, and others become
long live memory cells
74

75. Part Three b
75

76. LYMPHOID TISSUES
Consists of dense accumulations of lymphocytes
 Lymphoid organs are anatomical entities
consisting chiefly of lymphoid tissues
 The lymphoid tissues are typically located at sites
that provide a possible route of entry of pathogens
or sites liable to infections
76

77. Lymphoid organs and Tissues
77

78.  Are of two types – Primary and secondary
 Primary lymphoid tissues – (also known as central)
 Responsible for initial development and maturation of
immune cells; Ag-reactive cells
 Lymphocytes derived from BM stem cells develop in the
primary lymphoid tissues
 Thymus gland – T cell maturation
 Seeded by pre-T cells from BM
 Bone marrow – B cell maturation
 Can be referred to as Bursa equivalent
 Bursa of Fabricius in Birds – B cell maturation
 Birds lack a bone marrow, need to have lighter bones for flight
78

79.  T cell selection
 Based on MHC/Ag complex recognition
 Recognize MHC/Non self AG complexes
 Recognize MHC/Self Ag complexes
 Do not recognize MHC/Ag complexes
 Athymic condition
 Natural
 Other
Thymus
79

80.  Structure
 Microscopic
 Less well defined than thymus
 Role of stromal cells
 Function
 Hematopoiesis
 B cell maturation
 B cell selection
 Puts out mature, naive B cells
Bone Marrow
80

81. Key Concepts in lymphocyte
development
 Lymphocyte development-A process of differentiation
of lymphoid progenitor cells into mature lymphocytes
(T & B).
 Rearrangement and expression of Ag receptor genes
are associated with lymphocyte development.
 Selection events are involved in preserving cells w/
correct Ag receptors and eliminating dangerous cells
w/ self-recognition Ag receptor
 Proliferation in the early lymphocyte development is
stimulated by IL-7.
81

82. Stages of Lymphocyte Development
82

83. Checkpoints in Lymphocyte Development
83

84. Features of T lymphocyte development
1. Maturation of T Lymphocytes development
- Sequential Rearrangement & expression of TCR genes
- Selection & proliferation of T cell repertoire
2. Selection of the mature T cell repertoire occurs in Thymus
- Positive selection => Self MHC-restricted
- Negative Selection => Self Ag-MHC/high avidity =>
Apoptosis => Central Tolerance
3. CD4 & CD8 are surface markers for differentiation of
Thymocytes (immature T cells).
84

85. Stages of T lymphocyte development-I
85

86. T Lymphocyte Maturation in the Thymus
86

87. Features of B lymphocyte development
1. Maturation of B Lymphocytes development
- Rearrangement & expression of Ig gene in a precise order
- Selection & proliferation of pre-B cells via pre-Ag receptor
2. Selection of the mature B cell repertoire
- Self Ag => Affect the strength of the BCR signal
- Immature B cells => self Ag/high avidity => Receptor editing =>
Additional L chain recombination => Not Self-reactive
=> Fail to receptor editing => Apoptosis
3. During this maturation, B cell lineage cells go through
distinct stages => A specific Ig gene expression
=> Distinct surface markers
4. At Pre-B cell stage, H chain recombination occurs first and
associates with Surrogate light chains (l5 & VpreB).
- l5 & VpreB are similar to k & l light chains but invariant
- form pre-B cell receptor => Development
87

88. Stages of B lymphocyte development
88

89. Secondary lymphoid tissues
 Sites for Ag contact and immune response (effector
response)
 Positioned strategically throughout major parts of the
body for defense and maximum interaction of the ag
and the immune cells
 Mature (but naïve, immunologically virgin)
lymphocytes seed secondary L tissues to interact with
the antigen
 Recirculation is through the lymphatic system and
passage is through the High Endothelial Venules (HEVs)
– a post capillary system for homing of circulating
lymphocytes
89

90.  Expansion of lymphatic system, separate from blood
circulation.
 Small, flattened oval or bean shaped organs situated
in the course of collecting lymph vessels
 Structure
 Gross
 Bean-shaped structures
 Drains major segments of lymphatic system
Lymph Nodes
90

91.  Structure
 Microscopic
 Major cell types
 Lymphocytes
 Macrophages
 Dendritic cells
 Cortex/paracortex/medulla
 Follicles
 Primary
 Secondary
Lymph Nodes
91

92. 92

93. 93

94.  Function
 1st line of response to antigens
 Secondary follicle (Germinal center) is site of B cell
proliferation, mutation, differentiation
 Specificity is high
 >90% of B cells die through apoptosis
 After Ag stimualtion lymphocyte numbers up by 50X in
efferent lymphatic vessel
 Lympadenopathy
Lymph Nodes
94

95.  Similar to lymph nodes but part of blood circulation. Plays a role
of discriminatory filter
 Collects blood-borne Ags- a site of IR for blood-borne pathogens.
 It also filters aged blood cells for recycling
 Structure
 Gross
 Ovoid organ in upper left quadrant of abdomen
 Microscopic
 Compartmentalized
 Red pulp
 White pulp
 Periarticualr lymphoid sheath
 Site of Ag presentation
 Major cell types
 Lymphocytes
 Macrophages
 Dendritic cells
 RBCs
Spleen
95

96. 96

97.  Function
 Filters out older RBCs
 Responds to Ag in circulatory system
 Produces activated B cells
 Splenectomy
Spleen
97

98.  Mucosa-associated Lymphoid Tissue (MALT)
 Specialized mucosal immune system to protect mucosal
surfaces which come in contact with the outside world
 Differs from other sec. L tissues in the following
 Adapted to sampling ags from mucosa not blood or lymph
 Biased to prodn of IgA – the major secreted ab
 Tend to have its own lymphocytes which recirculate to the
MALT (using special homing receptors) rather than back to
the systemic lymphocyte pool
 Comprises of Tonsils, GALT and BALT
98

99.  Tonsils – Accumulations of lymphoid tissue
surrounding the openings of the digestive and
respiratory tracts
Tonsils plus smaller lymphoid tissue found between
them are also called Waldeyer’s ring
Lack afferent lymph vessels
Surface heavily fenestrated to enable sampling and
contact with the antigens
99

100.  Gut-associated Lymphoid Tissue (GALT)
Found throughout the GI tract
Most prominent accumulations occur in the ileum in
the form of Payer’s patches in the intestines and also in
the appendix
Contact with antigen is facilitated by epithelium cells
with deeply invaginated basal surface (the microfold
or M-cells)
Immune cells enter the invagination to contact the
endocytosed and processed antigens.
100

101. 101

102.  Bronchus-associated Lymphoid Tissue (BALT)
 Is analogous to GALT
 Has organized component of B and T cell-rich areas
following the airways
 Also a diffuse component in the mucosal connective
tissue (laminar propria) of the airways.
102

103.  Associated with intestines
 Responds to Ag
 Role in GI immune response
Appendix
103

104. Part Four
104

105. Major Histocompatibility Complexes
(MHC)
 MHC Act As Antigen Presenting Structures
 In Human MHC Is Found On Chromosome 6
 Cluster of genes found in all mammals
 Its products play role in discriminating self/non-self
 Participate in both humoral and cell-mediated immunity
 Referred to as HLA complex
 In Mice MHC Is Found On Chromosome 17
 Referred to as H-2 complex
 All cells except mature RBCs are genetically marked with
histocompatibility antigens on the membrane surface.
 Also called human leukocyte antigens (HLAs).
 The histocompatability antigens are coded for a group of
genes called MHC located on chromosome 6.
105

106. MHC
MHC genes produces 3 classes of MHC
molecules:
 Class I MHC genes
 Glycoproteins expressed on all nucleated cells
 Major function to present processed Ags to TC cells
 Class II MHC genes
 Glycoproteins expressed on M, B-cells, DCs
 Major function to present processed Ags to TH
 Class III MHC genes
 Products that include secreted proteins that have immune
functions. E.g. Complement system, inflammatory molecules
106

107. MHC Genes are Polymorphic
 MHC Products Are Highly Polymorphic
 Vary considerably from person to person
 However, Crossover Rate Is Low
 0.5% crossover rate
 Inherited as 2 sets (one from father, one from mother)
 Haplotype refers to set from mother or father
 MHC Alleles Are Co-dominantly Expressed
 Both mother and father alleles are expressed
 Inbred Mice Haplotypes Are Designated With Italic
Superscript
 Ex. H-2b
 Designation refers to entire set of H-2 alleles
107

108. Class I, II and III MHC
 Class I MHC Genes Found In Regions A, B and C In Humans
(K and D In Mice)
 Class II MHC genes found in regions DR, DP and DQ in
humans (IA and IE In Mice)
 Class I and Class II MHC share structural features
 Both involved in APC
 Class III MHC have no structural similarity to Class I and II
 E.g. TNF, heat shock proteins, complement components
108

109. 109

110. Inheritance of HLA Haplotypes
110

111. Class I MHC Molecule
 Produced by all cells but not RBCs.
 Picks up cytoplasmic peptides and
transports them to membrane.
 Killer T cells (cytotoxic) interact with
antigens.
 Co receptor CD8 permits each type of T
cell to interact only with a specific class of
MHC molecules.
111

112. Class I MHC Molecule
 Comprised of 2 molecules
  chain (45 kDa), transmembrane
 2-microglobulin (12 kDa)
 Non-covalently associated with each oth
 Association Of  Chain and 2 Is Required For Surface
Expression
  Chain Made Up Of 3 Domains (1, 2 and 3)
 2-microglobulin Similar To 3
 1 And 2 Form Peptide Binding Cleft
 Fits peptide of about 8-10 a/a long
 3 Highly Conserved Among MHC I Molecules
 Interacts with CD8 (TC) molecule
112

113. Class II MHC Molecule
 Comprised of  and  chains
  chain and  chain associate non-covalently
  and  chains Made Up Of Domains
 1 and 2 ( chain)
 1 and 2 ( chain)
 1and 1 Form Antigen Binding Cleft
  and  Heterodimer Has Been Shown To Dimerise
(consisting of two structurally similar monomers joined by bonds that can
be either strong or weak, covalent or intermolecular. The term homodimer is
used when the two molecules are identical (e.g. A-A) and heterodimer when
they are not (e.g. A-B)).
 CD4 Molecule Binds 2/2 domains
113

114. Class II MHC Molecules
 Produced only on antigen-presenting cells and B cells
 Appear only on cell membrane when cell is processing
antigens.
 Activate T cells.
 Helper T cells react with antigens.
 Coreceptor CD4 interact with only a specific class of
MHC molecule.
114

115. 115

116. 116

117. 117

118. Class I And II Specificity
 Several Hundred Allelic Variants Have Been Identified
In Humans
 However, up to 6 MHC I And 12 MHC II Molecules Are
Expressed In An Individual
 Enormous Number Of Peptides Needs To Be
Presented Using These MHC Molecules
 To Achieve This Task MHC Molecules Are Not Very
Specific For Peptides (Unlike TCR and BCR)
 Promiscuous Binding Occurs
 A peptide can bind a number of MHC
 An MHC molecule can bind numerous peptides
118

119. Class I And II Diversity And
Polymorphism
 MHC is one of the most polymorphic complexes known
 Alleles can differ up to 20 a/a
 Class I Alleles in Humans: 240 A, 470 B, 110 C
 Class II Alleles in Humans: HLA-DR 350 , 2 !
 HLA-DR
  genes vary from 2-9 in different individuals!!!,
 1  gene ( can combine with all  products increasing
number of APC molecules)
 DP (2 , 2 ) and DQ (2 , 3 )
119

120. 120

121. Class I MHC Peptides
 Peptides presented thru MHC I are endogenous Proteins
 As few as 100 Peptide/MHC complex can activate TC
 Peptide features
 size 8-10 a/a, preferably 9
 Peptides bind MHC due to presence of specific a/a found at
the ends of peptide. Eg. Glycine @ Position 2
121

122. Class II MHC Peptides
 Peptides presented thru MHC II are exogenous
 Processed thru endocytic pathway
 Peptides are presented to TH
 Peptides are 13-18 a/a long
 Binding is due to central 13 a/a
 Longer peptides can still bind MHC II by looping
 MHC I peptides fit exactly, not the case with MHC II
peptides
122

123. 123

124. MHC Expression
 Expression is regulated by many cytokines
 IFN, IFN, IFN and TNF Increase MHC expression
 Transcription factors that increase MHC gene expression
 CIITA (Transactivator), RFX (Transactivator)
 Some viruses decrease MHC expression
 CMV, HBV, Ad12
 Reduction of MHC may allow for immune system evasion
124

125. Part Five
125

126.  Immunogen A substance that induces a specific immune
response.
 Antigen (Ag) A substance that reacts with the products of
a specific immune response.
 Hapten A substance that is non-immunogenic but which
can react with the products of a specific immune response.
Haptens are small molecules which could never induce an
immune response when administered by themselves but
which can when coupled to a carrier molecule. Free
haptens, however, can react with products of the immune
response after such products have been elicited. Haptens
have the property of antigenicity but not immunogenicity.
126

127.  Adjuvants Substances that can enhance the immune
response to an immunogen are called adjuvants. The use of
adjuvants, however, is often hampered by undesirable side
effects such as fever and inflammation.
 Superantigens
 When the immune system encounters a conventional T-
dependent antigen, only a small fraction (1 in 104 -105) of
the T cell population is able to recognize the antigen and
become activated (monoclonal/oligoclonal response).
However, there are some antigens which polyclonally
activate a large fraction of the T cells (up to 25%). These
antigens are called superantigens
127

128.  Examples of superantigens include:
 Staphylococcal enterotoxins (food poisoning),
 Staphylococcal toxic shock toxin (toxic shock syndrome),
 Staphylococcal exfoliating toxins (scalded skin syndrome)
and
 Streptococcal pyrogenic exotoxins (shock).
 Although the bacterial superantigens are the best studied
there are superantigens associated with viruses and other
microorganisms as well.
 The diseases associated with exposure to superantigens are,
in part, due to hyper activation of the immune system and
subsequent release of biologically active cytokines by
activated T cells.
128

129. FACTORS INFLUENCING IMMUNOGENICITY
 Are grouped into three categories
1) Factors associated with the immunogen
2) Factors associated with the host
3) Factors associated with mode of administration of the
immunogen
129

130. 1. Immunogen factors
 Foreignness - The immune system normally
discriminates between self and non-self such that only
foreign molecules are immunogenic.
 Size - There is not absolute size above which a substance
will be immunogenic. However, in general, the larger the
molecule the more immunogenic it is likely to be.
 Chemical Composition - In general, the more complex
the substance is chemically the more immunogenic it will
be. The antigenic determinants are created by the primary
sequence of residues in the polymer and/or by the
secondary, tertiary or quaternary structure of the molecule.
130

131.  Physical form In general particulate antigens are
more immunogenic than soluble ones and denatured
antigens more immunogenic than the native form.
Degradability Antigens that are easily phagocytosed
are generally more immunogenic. This is because for
most antigens (T-dependant antigens, see below) the
development of an immune response requires that the
antigen be phagocytosed, processed and presented to
helper T cells by an antigen presenting cell (APC).
131

132. 2. Host factors
 Genetic Factors: Some substances are immunogenic in
one species but not in another. Similarly, some substances
are immunogenic in one individual but not in others (i.e.
responders and non-responders). The species or
individuals may lack or have altered genes that code for the
receptors for antigen on B cells and T cells or they may not
have the appropriate genes needed for the APC to present
antigen to the helper T cells.
 Age: Age can also influence immunogenicity. Usually the
very young and the very old have a diminished ability to
mount and immune response in response to an
immunogen.
132

133. 3. Mode of administration factors
 Dose - The dose of administration of an immunogen can
influence its immunogenicity. There is a dose of antigen
above or below which the immune response will not be
optimal.
 Route - Generally the subcutaneous route is better than
the intravenous or intra-gastric routes. The route of antigen
administration can also alter the nature of the response
 Other routes: IP – Intra-peritoneal, IM – Intra-muscular, IV – Intra-venous,
ID – Intra-dermal and SC – Sub-cutenous
 Adjuvants - Substances that can enhance the immune
response to an immunogen are called adjuvants. The use of
adjuvants, however, is often hampered by undesirable side
effects such as fever and inflammation.
133

134. Types of Antigens
T-independent
 Polysaccharides
• Properties
– Polymeric structure
– Polyclonal B cell
activation
• Yes -Type 1 (TI-1)
• No - Type 2 (TI-2)
– Resistance to
degradation
• Examples
– Pneumococcal polysaccharide, lipopolysaccharide
– Flagella
134

135. Types of Antigens
T-dependent
 Proteins
• Structure
• Examples
– Microbial proteins
– Non-self or Altered-self
proteins
135

136. Hapten-carrier conjugates
 Definition
Native determinants
Haptenic determinants
• Structure
– native
determinants
– haptenic
determinants
136

137. Hapten (incomplete antibody)
 is a molecule that cannot induce an immune
response by itself but can react with specific
immune response (antibody).
 Nevertheless, haptens can induce a response if
combined with larger molecules (normally
proteins) which serve as a carrier.
 Haptens are usually small.
 Many drugs, (e.g. penicillin) are haptens, and the
catechol in the plant oil that causes poison oak and
poison ivy is also a hapten.
137

138. hapten
No antibodies
produced
Antibody against
epitope on antigen
protein
Immunogenic antigen
epitope
epitope
hapten
Immunogenic antigen as carrier for hapten
Antibody against
epitope on antigen
Antibody against
epitope on hapten
138

139. Antigenic Determinants (Epitopes)
 Epitopes are – small chemical groups on the antigen
molecule that can elicit and react with antibody.
 An antigen can have one or more determinants. Most
antigens have many determinants; i.e., they are
multivalent.
139

140. 140

141. 141

142. Antigen epitopes
Antigen
epitopes
142

143. 143

144. Antigenic Determinants
Recognized by B cells and Ab
 Composition
 Proteins, polysaccharides, nucleic acids, haptens
 Sequence (linear) determinants
 Conformational determinants
 Size
 4-8 residues
 Number
 Limited (immunodominant epitopes)
 Located on the external surfaces of the Ag
144

145. Antigenic Determinants Recognized by T cells
 Composition
 Proteins (some lipids)
 Sequence determinants
 Processed
 MHC presentation (lipid presentation by MHC-like CD1)
 Size
 8 -15 residues
 Number
 Limited to those that can bind to MHC
145

146. A typical antigen:antibody reaction: gram-
negative bacterial pathogen may have several
antigens, or immunogens (flagella, pili and cell wall)
146

147. Antigenic structure of a bacterium
147

148. H
N
RN
A
Protein
М2
Protein
М1
Lipid
membrane
Antigenic structure of a virus
148

149. Antigenic properties of bacteria, toxins,
rickettsia and viruses that are used in the
practice of reproducing artificial immunity
against infectious diseases (Vaccines), are of
most practical importance.
149

150.  When the antigenic structures of the host are
similar to those of the causative agent,
the host is incapable of producing immunity, as
the result of which the disease follows a severer
course.
It is possible that in individual cases the carrier
state and inefficacy of vaccination are due to the
common character of the microbial antigens and
the antigens of the person's cells.
150

151.  It has been established that human erythrocytes
have antigens in common with staphylococci,
streptococci, the organisms of plague, E. coli.
Salmonella paratyphi, Shigella organisms, smallpox
and influenza viruses, and other causative agents
of infectious diseases.
 Such a condition is called antigenic mimicry.
151

152. Superantigens
 When the immune system encounters a conventional
T-dependent antigen, only a small fraction (1 in 104 -
105) of the T cell population is able to recognize the
antigen and become activated
(monoclonal/oligoclonal response).
 However, there are some antigens which polyclonally
activate a large fraction of the T cells (up to 25%).
 These antigens are called superantigens
152

153. Superantigens
Conventional Antigen
Monoclonal/Oligoclonal
T cell response
1:104 - 1:105
Superantigen
Polyclonal T cell response
1:4 - 1:10
153

154. Superantigens
 Superantigens- Bind simultaneously to the Vβ domain of a
T-cell receptor and to the α chain of a class II MHC
molecule (outside of TCR cleft)
 Exogenous superantigens-- soluble proteins secreted by
bacteria (I.e., staphylcoccal enterotoxins, toxic shock
syndrome toxin, exfoliative-dermatitis toxin, mycoplasma-
arthritidis supernatant and streptococcal pyrogenic exotoxins.
 Endogenous superantigens-- cell-membrane protein encoded
by certain viruses that infect mammalian cells. These viral
proteins are called minor lymphocyte stimulating (Mls)
determinants.
• Since superantigens bind outside of the TCR antigen-
binding cleft any T cell expressing a particular V$ sequence
will be activated (polyclonal response).
154

155. Superantigens
 Definition
 Examples
 Staphylococcal enterotoxins
 Staphylococcal toxic shock toxin
 Staphylococcal exfoliating toxin
 Streptococcal pyrogenic exotoxins
 Although the bacterial superantigens are the best studied there are
superantigens associated with viruses and other microorganisms as
well.
 The diseases associated with exposure to superantigens are, in part,
due to hyper activation of the immune system and subsequent
release of biologically active cytokines by activated T cells.
155

156. Determinants Recognized by the Innate
Immune System
 Adaptive Immune System – Discrete Determinants
 Reacts with a specific pathogen
 Innate Immune System – Broad Molecular Patterns
 Reacts with a variety of pathogens
156

157. Determinants Recognized by the Innate
Immune System
 PAMPs – Pathogen Associated Molecular Patterns
 PRRs – Pattern Recognition Receptors
157

158. PAMP PRR
Biological
Consequence of
Interaction
Microbial cell wall
components
Complement Opsonization;
Complement
activation
Mannose-
containing
carbohydrates
Mannose-binding
protein
Opsonization;
Complement
activation
Polyanions Scavenger receptors Phagocytosis
Lipoproteins of
Gram+ bacteria
Yeast cell wall
components
TLR-2 (Toll-like
receptor 2)
Macrophage
activation;
Secretion of
inflammatory
cytokines
158

159. PAMP PRR
Biological
Consequence of
Interaction
Double stranded
RNA
TLR-3 Production of
interferon
(antiviral)
LPS
(lipopolysaccharide
of Gram– bacteria
TLR-4 Macrophage
activation;
Secretion of
inflammatory
cytokines
Flagellin (bacterial
flagella)
TLR-5 Macrophage
activation;
Secretion of
inflammatory
cytokines
159

160. PAMP PRR
Biological
Consequence of
Interaction
U-rich single
stranded viral RNA
TLR-7 Production of
interferon
(antiviral)
CpG containing
DNA (Cytosine-
phosphate-Guanine)
TLR-9 Macrophage
activation;
Secretion of
inflammatory
cytokines
160