This presentation provides an overview of cell and humoral immunity, two important components of the immune system. Cell-mediated immunity is mediated by T cells, while humoral immunity is mediated by B cells and antibodies. The presentation discusses the different types of cells and molecules involved in each type of immunity, as well as the roles they play in protecting the body from infection.

1. Immune Responses
Cell and Antibody Mediated
Presented by
Prashant Yadav
M.Sc. Microbiology

2.  Immunology is the study of specific immunity and how the immune system
responds to specific infectious agents.
 The immune system consists of various cells, especially lymphocytes, and organs
such as thymus gland, which participate in providing the host with specific
immunity to infectious agents.
 The immune system protects an individual against invasion by foreign bodies
specially microbial agents and their toxic products
 The integrity of the body is maintained by the multiple defense systems including
immune response.
 Immunity produced by such types of defenses is called as nonspecific immunity. In
contrast, specific (or acquired ) immunity is the ability of a host to mount a defense
against particular infectious agent by physiological responses specific to that
infectious agent.
Introduction

3. There are two types of immune response :
Primary Immune Response
Secondary Immune Response
Primary Immune Response
 A primary immune response occurs when the body immune response encounters an
antigen for the first time .
During the immune response, the body learn to recognize the antigen , produce
antibodies against the antigen ,and induce a long-term memory response against the
antigen.
Memory induction is enabled by the production of immune memory lymphocytes
The response induced involved the activation of naïve B-cell and T-cells.

4. Secondary Immune Response
 This is the subsequent immune response after the primary immune response ,
also know as anamnestic immune response.
The response that occurs the second or third or fourth etc.
This responses is mediated by the memory lymphocytes that were produced
during the primary response .
This response has a very short sharp lag phase which means that both lag and
exponential phases take place ,normally within a few days.
This is because of the antigen specific memory T and B-cells produced during
the primary response.

6. Four phases of immune response.
Lag phase
 It is also known as the latent phase
 This leads to the activation of naive B-cells that produce antibodies to counter
the antigen
 This phase takes a week, activating the specialized B and T cells.
Exponential Phase
 This is the phase of a rapid increase in antibody production by the differentiated
plasma cells.
 The increase in antibody production is because of the large number of plasma
cells.

7. Plateau Phase
 This is a steady phase where the antibody level remains constant to
maintain the levels of antibody replenishing and production. This means
that the antibodies that get used up equal the antibodies that are
produced.
Decline phase
 This phase involves a decrease in antibody levels due to the decline in
plasma cell numbers which are dying out of exhaustion of antibody
production.
 During this phase, there are no new plasma cells being produced because
the antigen or immunogen has been eliminated from the system.

8. HELPER T CELLS(TH)
 T cells are so called because they are predominantly produced in the thymus.
 A type of immune cell that stimulates killer T cells, macrophages, and B cells
to make immune responses.
 A helper T cell is a type of white blood cell and a type of lymphocyte. Also
called CD4-positive T lymphocyte.
 They recognise foreign particles (antigen) by a surface expressed, highly
variable, T cell receptor (TCR).
 There are two major types of T cells: the helper T cell and the cytotoxic T cell.
 As the names suggest helper T cells ‘help’ other cells of the immune system,
cytotoxic T cells kill virally infected cells and tumours.

9. Activation of Helper T Cells
Activation of TH cells requires generation of three specific signals:
1. Antigen-specific signal: It involves binding of antigenic peptide present in the groove
of MHC-II(major histocompatibility complexes) on APCs(antigen presenting cell) to
TCR (T cell receptor) present on surface of TH cells. CD4 molecules of TH cells also
interact with β2 domain of MHC-II.
2. Costimulatory signal: It involves binding of CD28 molecule on TH cells to B7
molecules on APCs.
3. Cytokine signal: APCs (macrophages) secrete interleukin-1 (IL-1) which acts on TH cells.

10. Effector TH Cells
 They not only help activate B cells to secrete antibodies and macrophages to destroy
ingested microbes.
 They also help activate cytotoxic T cells to kill infected target cells.
 They further differentiate into either TH1 or TH2 subsets.
 TH1 cells produce IL-2, interferon-γ (IFN- γ) and tumor necrosis factor-β (TNF-β).
 TH2 cells secrete IL-4, IL-5, IL-6, IL-10 and IL-13. They activate the B cells to
transform into plasma cells which in turn secrete antibodies.

12. The Immune response can be two types :
 Humoral ( antibody-mediated)
 Cellular (cell-mediated)
abbreviated as AMI and CMI
The humoral immune system involves
antibodies that are dissolved in the blood
plasma and lymph. These antibodies are
produced by lymphocytes called B cells
The cellular, or cell-mediated immune
system depends on lymphocytes called T
cells, which are located both in the blood and
in the lymphoid tissues. T cells do not secrete
antibodies into body fluids.

13. Immune Responses: Cell-mediated and Antibody-mediated

14. Humoral ( antibody-mediated)
Introduction
The word “Humors” means body fluids, and humoral immunity is derived from
immunity mediated by extracellular fluids which contain macromolecules such as
antimicrobial peptides, antibodies, etc.
This response deals with antigens from pathogens circulating or are outside the infected
cells.
Various defence mechanisms such as opsonization, neutralization, and complement
system activation are utilized by antibodies to attack and destroy any invading pathogens
such as bacteria, viruses, fungi, and toxins.

15.  B cell lymphocytes which are a type of
immune cells, produce antibodies and help in
detection of specific antigens.
 Antibodies are highly specific. Specific
antibodies are produced for their respective
antigens.
 Antibodies are proteins that identify and
neutralize antigens as they contain specific
antigen-binding sites which attach to the
particular target antigens.
Humoral Immune System Functions

16.  Upon effective attachment to the desired antigen, they either stop the normal antigen
binding process or initiate the production of autoimmune cells, directly destroying or
removing the target antigen
 B cells are fewer in number than T cells as these account for about 20% of the blood
lymphocytes in the body, whereas the rest are T cells.
 These cells have receptors on the surface known as B cell receptors.
 The B cell receptor is a macromolecular complex that is built with the help
of IgM and IgD.

17. Antibody Production
 The bone marrow is a soft sponge-like organ that is present inside the bones of the body.
 Stem cells within the bone marrow produce red blood cells, white blood cells, and platelets
that are actively involved in the body's immune system.
 B lymphocytes or B cells, produced by the bone marrow, are the major constituents in the
humoral mediated immunity and circulate in the body through the lymph or blood.
 They mature in the secondary lymphoid tissues after they are produced (the tonsils, lymph
nodes, spleen, and mucosa-associated lymphoid tissue).
 B cells produce antibodies that bind to the surface and remain there, allowing their exposed
antigen-binding site to pick up any potential petitions or foreign body signals. This attached
form of an antibody is called immunoglobulin.

18.  When the antigen-specific to antigen-binding site of the immunoglobulin binds to
the naive or memory B cells.
 This B cell then matures and releases antigen-specific antibodies. It is now known
as a plasma or a memory B cell for that specific antigen.
 The antibodies from this plasma cell are called monoclonal antibodies, and they are
produced only on exposure to that antigen.
 Thus upon any secondary exposure to that particular antigen, the memory B cell
enables the body to respond quicker and stronger to that antigen.
 Synthesis and production of antibodies typically is dependent on complex
interaction of three cells: (a) macrophages, (b) helper T cells, and (c) B cells

19. Activation of B Cells
Antigens that activate B cells fall into two categories.
1. Most antigens are thymus-dependent (TD); they activate B cells indirectly via
activation of T cells. TD antigens are processed by APCs, presented to TH cells
following which the activated TH cells secrete cytokines that in turn activate the B
cells
2. The thymus independent (TI) antigens (e.g. bacterial capsule) are not processed by
APC. They can directly activate B cells without the help of T cell induced
cytokines ,TD antigens induced activation of B cell.

20. Antigen Presentation of B Cells to Activated TH Cells
 That occurs is recognition of microbial antigen
(TD antigen) by B cell membrane
immunoglobulin receptors (mIg) followed by
receptor-mediated endocytosis of antigen.
 Then the antigen is processed into smaller
antigenic peptides that are presented in
complex with MHC-II to activated TH cells (by
endocytic pathway). This leads to induction of
three signals.

21. Signal Induction
Signal 1: It is induced by the cross linking of
IgM on B cell membrane with the microbial
antigen .
Signal 2: It is an additional signal provided by
binding of CD40 on B cell with CD40L (ligand)
on activated TH cells .
Signal 3: It is usually a cytokine stimulus.
Cytokines produced by the activated TH cells
bind to specific cytokine receptor on B cells.

22. Signal Transduction
Signal transduction is initiated by the B cell receptor (BCR). The BCR comprises of
two parts-:
1. Antigen-binding membrane Ig
2. Ig-α/Ig-β heterodimer. ™
Antigen cross linkage to membrane Ig, the Ig-α/Ig-β heterodimer is activated and in
turn transmits the signal, ultimately leading to activation of B cells.
Proliferation and Differentiation of B Cells
The naive B cells, released from bone marrow go and house in the B cell areas of
peripheral lymphoid organs (e.g. cortex of lymph node and marginal zone of spleen).

23. Germinal Center
Germinal center (GC) B cells cycle
between the dark zone (DZ) and light
zone (LZ) during antibody affinity
maturation.
The dark zone contains large
centroblasts that are rapidly
proliferating and undergoing somatic
mutation of their antibody variable
region genes.

24. Light Zone
 Binding of centrocytes to follicular dendritic
cells:
 The centrocytes with high affinity membrane
Ig undergo maturation by binding to a special
type of dendritic cell called follicular
dendritic cell
 Then the mature centrocytes undergo class
switch over

25. Differentiation of centrocytes into plasma cells and memory cells
 The selected centrocytes further undergo differentiation into effector cells (plasma
cells) and memory cells in the light zone of germinal center „
 Plasma cells are large antibody-secreting cells.
 Produce secretory Ig enormously, but do not synthesize membrane Ig.
 They do not have MHC-II molecules and do not undergo further class switch over „
Memory cells bear high affinity membrane Ig molecules of all classes as compared to
naive B cell that bear only low affinity IgM or IgD membrane Ig.
 They are long lived cells which respond to the secondary antigenic stimulus.

26.  Cell-mediated immune responses involve the destruction of infected cells by
cytotoxic T cells, or the destruction of intracellular pathogens by macrophages.
 The activation of naive T cells in response to antigen.
 Their subsequent proliferation and differentiation, constitutes a primary immune
response.
 Cell-mediated immunity is an immune response that independent antibody but
dependent on the recognition of antigen by T cells to eliminate intracellular
pathogens.
 It's type IV hypersensitivity reaction.
Cell-mediated immune responses

27. Cells contribute to cell- mediated immune response:
A. Antigen – specific cells:
 CD8+ Cytotoxic T- lymphocytes (Tc cells or CTLs);
 Cytokine- secreting CD4+ TH cells mediated delayed type
hypersensitivity (DTH)
B. Antigen – nonspecific cells:
NK cells;
DC;
Non-lymphoid cell types…Macrophages, neutrophils and
eosinophils

28. o However, activation of cells to carry out killer functions requires cooperation of
different cell types (cooperation of Cellular and humoral immunity).
o Cells such as Mq, NK, neutrophil and eosinophil can use antibodies to
recognize and presented Ag for killing.
o The same time chemotactic peptides generated by the activation of complement
in response to Ag-Ab complexes can contribute to assembling cells required for
a cell- mediated response.

29. Role of CMI
 Provides immunity against microbes residing in intracellular milieu:
 CMI remains the only effective immune response. Examples include all
viruses, some bacteria (Mycobacterium, Chlamydia and Rickettsia), some
parasites (Plasmodium, Leishmania, Trypanosoma and Cryptosporidium)
and some fungi (Pneumocystis)
 Provides immunity against tumor cells and other damaged and altered cells.
 Mediates delayed hypersensitivity (type IV hypersensitivity).
 Plays key role in transplantation immunity and graft-versus-host (GVH)
reaction.

30. Cytotoxic T Lymphocytes
 CD8 cytotoxic T lymphocytes (CTL or Tc) are the principal effector cells of CMI.
 The destruction of target cells such as virus infected host cells and tumor cells.
 The target host cells (by cytosolic pathway) and presented along with MHC class I
molecules.
 Activated TC in turn secretes cytotoxic enzymes that lyse the target cells.

31. Activation of CTL (Cytotoxic T Lymphocytes)
Naive TC cells requires induction of at least three signals:
1. Antigen-specific signal: It is induced by binding of TCR-CD3 complex of
naive Tc cells to MHC I-peptide complex of target cells. CD8 of Tc cells
also interacts with α3 domain of MHC-I.
2. Costimulatory signal: CD28 of naive TC cells interacts with B7 molecule
on target cells.
3. Third signal: IL-2 (secreted by TH1 cell) acts on high affinity IL-2 receptor
on T.

32. Functions of CTL (Target Cell
Lysis)
The activated TC cells produce two types of lethal
enzymes; called perforins and granzymes
Perforins produce pores in the target cell
membrane; through which granzymes are
released inside.
Granzymes are serine proteases; they induce
cell death by apoptosis through caspase
pathway.
Activation of CTL

33. Natural Killer Cells
 NK cells for their non specific cytotoxicity make
up 5 -10% of the circulating lymphocyte
population, derived from bone marrow.
 These cells are involved in immune defenses
against viruses, intracellular pathogens and
tumors.
 NK cells produce immunologically important
cytokine.
 They play contributing roles in immune
regulation and influence.

34. Mechanism of NK Cell-mediated Cytotoxicity
 Natural killer cells are not MHC restricted.
 They directly recognize certain ligands (e.g. glycoproteins) present on the
surface of host cells like virus-infected cells or tumor cells.
 NK cells are capable of distinguishing normal host cells from the altered cells
 This is mediated by two types of receptors present on NK cell surface

35. Activation receptors-:
• Activating NK cell receptors include members of
the human Killer Immunoglobulin-like Receptor
(KIR) family or the mouse Ly49 family.
• When these receptors are engaged with ligands
present on the target cells; NK cells become
activated
Inhibitory receptors
• Inhibitory receptors regulate the strength of the
signal of activating receptors.
• Inhibitory receptors relay their inhibitory signal
through one or more immunoreceptor tyrosine-
based inhibitory motifs (ITIMs) present in their
cytoplasmic domains.

36. Antibody-dependent Cell-mediated Cytotoxicity (ADCC)
 ADCC makes use of the innate immune cells providing antitumor cytotoxicity
activated by antibodies linked to target cells.
 Natural killer (NK) cells are a small set of lymphocytes.
 Antibodies can bind to their specific antigens on the target cell surface via their
antigen-binding fragment (Fab) portions and interact with effector cells via their
fragment crystallizable region (Fc) portions.
 ADCC plays a role in several biological processes like autoimmune diseases,
antitumoral defence, anti parasitical defence, antiviral defence..

37.  NK cells secrete perforins, and granzymes.
Neutrophils release lytic enzymes
 Eosinophils can release lytic enzymes and
perforins; play an important role in providing
immunity against helminths
 Macrophages produce lytic enzymes and
TNF (Tumor necrosis factor).

38. Detection of CMI
 The mixed-lymphocyte reaction (MLR) is an in vitro system for assaying
T cell proliferation in a cell-mediated response.
 Cell-mediated lympho-lysis (CML) is another in vitro assay for testing the
cytotoxic function of effector cells of CMI.
 The graft-versus-host (GVH) reaction in experimental animals provides
an in vivo system for studying cell-mediated cytotoxicity