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This chapter primarily deals with characteristics of different types of antigens and the response of the animal body to these antigens. Specifically, the objective is to understand the effecters of humoral immune response (i.e., antibodies) and cell-mediated immune response. A brief understanding of immunologic tolerance is expected. Study of this chapter should enable one to answer questions on the following:
1. differentiation of thymus-dependent and thymus-independent antigens
2. immunogenic characteristics of the antigens
3. should be able to draw a diagram of basic immunoglobulin molecule and identify different parts and write the significance of those parts
4. characteristics of the 5 isotopes of immunoglobulins
5. generation of antibody diversity
6. distinguishing characteristics of various cells involved in cell- mediated immunity
7. importance of immunological tolerance.
17.1.1 Antigens and Antigenicity
Terms antigen and immunogen are often used synonymously. Antigens (immunogens) are defined as substances which:
(i) induce antibody production and/or specific cell-mediated responses and
(ii) specifically react with antibody induced by the immunogen.
Proteins are highly immunogenic when injected into animals in which they are not normal (self).
Polysaccharides and nucleic acids are also immunogenic but nucleic acids are only weakly immunogenic.
Small molecules whose Mol. wt. is below 2000 daltons do not induce antibody formation (Haptens), but when coupled to carriers elicit antibodies.
Antibodies produced in response to antigens react with only certain portions of antigens called antigenic determinants (epitopes).
17.1.2 Thymus-dependent and thymus-independent antigens:
B cells produce antibodies when they react with two different types of antigens:
1) T-dependent antigens - Many antigens require T cell recognition to help a B cell.
2) T-independent antigens- small number of antigens capable of activating B cells independently of T cell help. They are usually large polymeric molecules with repeating antigenic determinants. Eg. bacterial lipopolysacharide, ficoll, dextran, polymeric flagellin etc.
17.1.2.1 Requirements for Immunogenicity
Must be sufficiently foreign to the host; more foreign the better. Exceptions: thyroid tissue, brain tissue, eye lens and sperm.
1. Molecular size; larger the better.
2. Stability - more stable the better.
3. Complexity - simple polymers are poor antigens.
4. Degradability - inert molecules are poor antigens.
17.1.2.2 Mitogens
Mitogens in contrast to antigens are nonspecific. They are substances that activate or stimulate lymphocytes to undergo proliferative changes such as DNA synthesis, blast formation, etc. Plant mitogens are called lectins.
Example of mitogens:
B cell mitogens - antiimmunoglobulin serum, dextran sulfate, lipoplysaccharide (LPS).
T cell mitogens - Concanavalin A (Con A) , Phytohemagglutinin (PHA).
T and B cells - pokeweed mitogen (PWM), antilymphocytic serum.
Mitogens are commonly used to study lymphocyte proliferative responses in the laboratory.
17.1.2.3 Humoral Immunity
Structure and Function of Immunoglobulins:
Antibodies/immunoglobulins (Ig) are produced by plasma cells in response to antigenic stimulation.
17.1.2.3.1 Basic structure of the immunoglobulin molecule:
- four polypeptide chains
- two identical chains of mol. wt. 50,000 daltons (Heavy chains).
- two identical chains of mol. wt. 25,000 daltons (Light chains).
- chains are held together by disulfide bonds.
- two types of light chains - kappa and lambda; In a single immunoglobulin molecule they are either kappa or lambda but never a mixture.
The amino-terminal end region has sequence variability in both heavy and light chains and is referred to as VH and VL regions, respectively. Each of these regions contain three hypervariable regions (HV1, HV2, and HV3). The remaining portion is fairly constant. Constant portion of light chain is - CL region. Constant portion has three "domains" called CH1, CH2, and CH3. Hinge region is a segment of heavy chain between CH1 and CH2 domains.
Light (L) chains:
composed of approximately 220 amino acid residues and are encoded in 3 separate DNA segments in germ-line cells: (V)ariable gene, (J)oining gene and (C)onstant gene. When the germ line develops the V and J segments are joined but remain separate from the C gene.
Heavy (H) chains:
composed of approximately 440 amino acid residues. The formation of the H chain, is similar to L chain assembly except that the V region is formed by joining 3 rather than 2 DNA segments: (V)arible gene, (D)iversity gene, and (J)opening gene. This unit remains separate from the (C)onstant gene. The D gene has been identified in germ line and differentiating immune cells. The V-D-J genes join after undifferentiated immune cells begin to develop into antibody producing cells. (See hand out for structure of Ig molecule).
Constant Region:
apparently governs the diverse function of antibody such as the complement fixing site (CH2) domain, and cellular and tissue localization.
Sequences of genes reveal that coding regions (exons) induce antibody diversity by somatic shuffling of exons that code for antigen binding regions. Introns (non coding regions) are perhaps vestigial transposable elements inserted into exons. They are excised during gene transcription into RNA.
By arrangement of multiple C, J and V genes on the L chain and C, J, D and V on the H chain in various ways to produce numerous H and L chains, it is possible to make millions of immunoglobuins with only a few hundred genes.
17.1.2.3.2 Generation of immunoglobulin diversity:
Antibodies must provide enough different combining sites to recognize the millions of antigenic shapes in the environment.
Five mechanisms:
1. Multiple germ line V genes
2. VJ (light chain) and VDJ (heavy chain) recombinations
3. Recombinational inaccuracies
4. Somatic point mutation
5. assorted heavy and light chains
17.1.2.3.3 Significance of Immunoglobulins
Immunoglobulin (Ig) isotopes- IgG, IgM, IgA, IgD, IgE
The class and subclass (eg. IgG1, IgG2, IgG3, IgG4) of an Ig molecule is determined by its heavy chain type.
IgM
first antibody formed in the newborn vertebrate and first and major Ig in the primary immune response. Mol. wt. - approximately 900,000 daltons. Exists in pentameric form of five subunits held together by J chain. Second highest in serum concentration. Intravascular, efficient in complement activation, opsonization, agglutination and virus neutralization.
IgG
major Ig in serum; major Ig formed in secondary responses. escapes readily into extravascular spaces; can opsonize, agglutinate and precipitate antigen. Appears in increased amounts in sera after an initial lag on the first immunization. Is qualitatively different in secondary responses.
IgA
common form is dimer consisting of two identical units joined by J chain but may be seen as monomer or trimer also. Minor component in animal serum. Major Ig in external secretions in nonruminants (IgG1 in ruminants). Protects mucosal surfaces, mammary gland and eyes. Does not activate complement and is not an opsonin. When secreted by plasma cell in mucosa it binds to a receptor (secretory component) on epithelial cells. The complex is then called secretory IgA (sIgA).
Function of secretory component: protects IgA against proteolytic enzymes and faciltates transport of sIgA into secretions.
IgD
found on membrane of B lymphocytes; sensitive to proteolytic enzymes; precise function not known; concentration in serum very low; may serve as an antigen receptor and play a role in antigen-triggered clonal proliferation.
IgE
reagenic antibody - mediates type I hypersensitivity; binds to mast cells and basophils and together with antigen mediates release of vasoactive amines such as histamine, serotonin etc., which are the mediators of anaphylaxis.
17.1.2.3.4 Functions of antibody in infection:
-Toxin neutralization
-Microbial binding - prevents attachment, neutralization
-Cytotoxicity against virus-infected cells
-Opsonization (helps in phagocytosis)
-Complement fixation (lysis)
-Mast cell degranulation
17.1.2.4 Cell- mediated immunity
17.1.2.4.1 T cells
-act as coordinators of immune response
-effector cells in cell-mediated immunity
-express T cell antigen receptor (TCR)
TCR - two types: TCR1 ( and polypeptides)
TCR2 ( and polypeptides)
-both receptors are associated with a complex of polypeptides.
-CD3 complex (TCR/CD3 complex; expressed on all T cells).
The prefix CD stands for cluster designation.
T cells express several other surface molecules.
Two distinct poplations:
1. TH subset (helper subset) (CD4+): provides help to B cells and Tc/s cells.
2. Tc/s subset (cytotoxic/suppressor subset) (CD8+).
Acivation of T cells:
1. most T cells in the body are in the resting phase.
2. require two signals for activation - first signal is antigen and second signal is IL-1.
3. when they encounter antigen _ clonal expansion _ effector cells and memory cells.
Important: T cells do not recognize antigen directly - they recognize it on the surface of other cells (antigen presenting cells)) in association with major histocompatibility (MHC) molecules.
17.1.2.4.2 Antigen presenting cells (APC):
- TH cells recognize antigen when presented by an antigen presenting cell (APC) in association with MHC class II molecules.
- Majority of antigens require some internal processing (degradation) by APC before they are presented to T cells.
Examples of APCs: Macrophages, B cells, dendritic cells, langerhan's cells.
17.1.2.5.1 Tc/s cells:
- Tc/s cells recognize antigen in the context of class I MHC molecules.
- kill cells that express self-class I MHC molecules and foreign antigens, such as virus-infected cells.
- receive help from TH cells.
17.1.2.5.2 Killer cells:
- participate in antibody-dependent cell-mediated cytotoxicity (ADCC).
- recognize Fc of IgG bound to antigen on the target cell surface.
17.1.2.5.3 Natural killer cells (NK cells):
- recognize determinants expressed on neoplastic cells.
- derived from large granular lymphocytes (LGLs).
Monocytes and neutrophils also participate in ADCC.
Macrophages:
The term activated macrophage was originally used to describe profound microbicidal activity in infections with facultative intracellular parasites. This type of immunity is nonspecific in expression, i.e., macrophages harvested from an animal immunized to a specific bacterial infection displays increased microbicidal activity to a range of unrelated bacteria. While the microbicidal mechanism of macrophages is nonspecific, the acquisition of the mechanism is dependent on specific T cell stimulation.
Macrophages may be activated by:
a. products of activated lymphocytes,b. immune complexes,
c. complement components (C3b).
As a result of macrophage activation, they may
a. secrete enzymes.b. secrete neutral proteases able to cleve C3, degrade peptidoglycan, collagen, activate plasiminogen, and prostaglandins.
c. release complement components: C3, C5, C6and C7 and induce interferon production.
17.1.2.5.4 Immunological Tolerance
Tolerance is a specific immunosuppressive response to a particular antigen. The specific response is diminished or abolished but all other host immune responses are unaffected.
Innate tolerance: state of tolerance to self-antigens that develops during early fetal life.
Acquired tolerance: experimental manipulation of immune system of immature or adult animals that result in tolerance to specific antigens.
Example:
- Dizygotic cattle twins though different genetically may be chimeras because they shared the same placenta with anastomosis of placental blood vessels. These twins accept skin grafts from each other and tolerate each others genetically distinct blood cells.
Studies suggest that
- Tolerance is more easily established with T cells.
- Only T cells are made unresponsive by low dose tolerance.
- Both T and B cells are made unresponsive to high dose tolerance.
- T cells remain tolerant about 3 times longer than B cells.
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Veterinary Clinical Immunology
(Halliwell and Gorman). Immunology by Roitt et
al.
Veterinary Immunology
(Tizzard)
