Antigen And Antibody

ANTIGEN AND ANTIBODY Presented By

DR. DHIREN B. BHOI DEPT. OF GYNAECOLOGY AND OBSTETRICS COLLEGE OF VETERINARY SCI. & ANIMAL HUSBANDRY S. D. AGRICULTURAL UNIVERSITY GUJARAT (INDIA) E-mail:- [email protected]

Antigen

“Antigens are substances that induce a specific immune response and subsequently react with the products of a specific immune response.”

• The word, antigen, originated from the notion that they can stimulate antibody generation. We now know that the immune system does not only consist of antibodies. • An antigen is a molecule that stimulates an immune response. • The modern definition encompasses all substances that can be recognized by the adaptive immune system (i.e. T cell & B cell).

Characteristics of Antigen

1.IMMUNOGENICITY The capacity to stimulate the production of antibodies or cell-mediated immune responses. 2.ANTIGENICITY Ability to bind antibody 3. COMPLETE ANTIGEN Antigen has both immunogenicity and antigenicity.

4.INCOMPLETE ANTIGEN  Also known as hapten Antigen has only antigenicity.

Incomplete antigens have antigenic determinants,  They cannot induce immune responses one example of an incomplete antigen is a hapten, which is an artificial monovalent epitope)

TWO FUNCTION OF ANTIGEN 1. Immunogenicity “An ability of antigen which can stimulate the body evoke a specific immune response.” 2.Immunoreactivity “ An ability of antigen which can combine with corresponding Antibody or sensitized T lymphocyte. “

to

Structure of Antigen

Classification

According to their [1] Exogenous antigens origin

Exogenous antigens are antigens that have entered the body from the outside, for example by inhalation, ingestion, or injection. By endocytosis or phagocytosis, these antigens are taken into the antigen-presenting cells (APCs) and processed into fragments. e.g. foreign cells such as bacteria, virus

[2]

Endogenous antigens Endogenous antigens are antigens that have been generated

within the cell, as a result of normal cell metabolism, or because of viral or intracellular bacterial infection. e.g autologus antigen or autoantigen

[3] Autoantigens

An autoantigen is usually a normal protein or complex of proteins that is recognized by the immune system of animals suffering from a specific autoimmune disease. e.g. rheumatoid arthritis and immune thyroiditis

[4] Tumor antigens Tumor antigens are those antigens that are presented by the MHC I molecules on the surface of tumor cells. These antigens can sometimes be presented only by tumor cells and never by the normal cells. In this case, they are called tumor-specific antigens (TSAs) and typically result from a tumor specific mutation.

According to relative [1] xenoantigen

The prefix "xeno" means foreign or other. It comes from the Greek "xenos" meaning stranger, guest, or host. An antigen that is found in more than one species. An antigen is something that is capable of inducing an immune response.

[2] Alloantigen An antigen existing in alternative (allelic) forms in a species, thus inducing an immune response when one form is transferred to members of the species who lack it; typical alloantigens are the blood group antigens.

Other Ag. [1]Complete antigen antigens with both immunogenicity and antigenicity [2]hapten: antigens with antigenicity but not immunogenicity [3]Thymus depended and independed Antigen Thymus dependent antigen [TD-Ag] Require accession of T and APC to active B cell initial both humor and cell immunity induce many types of antibody induce immune memory

Thymus independent antigen, TI-Ag Directly active B cell independent T cell Only initial humor immunity and induce IgM not induce immune memory

TD antigen

TI-1 antigen

chemical property

protein

lipin Polysaccharides polysaccharides , glycans

Epitope type

T, B epitope reduplicative B bulk epitope reduplicative B epitope

B cell clone activated

Most B2

Neogenesis respond mouse and baby

TI-2 antigen

Mature and Mature B1 immature B1,B2 respond

Non respond

• Heterophile antigen  Common antigen owned by different germ lines  Forssman was the first to find that there were common antigen between cony pig organ and sheep erythrocytes , which was called Forssman antigen.

• Idiaotype antigen  TCR 、 BCR or Ig can induce antibody production too.

Superantigen (SAg)

 Antigens that can non-specifically stimulate a plenty of T/B cells and induce a very strong Ir with a extremely low concentration 

The mechanism of their action is different from that of common Ags

Heterogenous Ag (xenoantigen) “An antigen that is found in more than one species.”

(1) Microbial Ag  Surface antigen [“K” Ag] Pillus Ag [”F” Ag ] Somatic Ag  Flagellar Ag [ “H” Ag] (2) EXOTOXIN AND TOXOID EXOTOXIN: Produced by G+ve bacteria Strong immunogenicity and pathogenicity TOXOID: Under suitable condition exotoxin loss its toxicity without affecting it’s immunogenicity then exotoxin turned toxoid. e.g. tetanus toxoid (3) HETEROPHILE ANTIGEN Common antigen owned by different germ lines

ANTIGENIC EPITOPES Also known as Antigenic determinants. “It is the part of a macromolecule( large molecules) that is recognized by the immune system, specifically by antibodies, B cells, or T cells.” The part of an antibody that recognizes the epitope is called a paratope. TWO DIFFERENT EPITOPES

• B cell epitope, a portion • T cell epitope, the of antigen molecule that

region of antigen

is recognized by B cell

molecules that are

receptors (BCR).

recognized by T cell receptors (TCR).

TWO TYPE OF EPITOPE (A) Linear epitopes continuous and found in polysaccharides as well as in both native (nondenatured) and denatured proteins, especially fibrillar proteins. specificity depends upon primary sequence. typical size is 5-6 subunits in length (B) CONFORMATIONAL EPITOPE Discontinuous (involve multiple subunits, often located far apart in the primary sequence of the antigen molecule) and are thus found only in native (globular) proteins.

ADJUVANT “An adjuvant is an agent that may stimulate the immune system and increase the response to a vaccine, without having any specific antigenic effect in itself.” "An immunologic adjuvant is defined as any substance that acts to accelerate, prolong, or enhance antigen-specific immune responses when used in combination with specific vaccine antigens” CLSSIFICATION OF ADJUVANT 1.Freund’s adjuvant Complete Freund’s adjuvant (CFA)  Incomplete Freund’s adjuvant (IFA) 2.Liposome 3.Inorganic compound There are many adjuvants, some of which are inorganic (such as alum), that also carry the potential to augment immunogenicity

4.Cytokine 5.Biodegradable nanoparticles 6.Organic adjuvants organic adjuvants are more commonly used in animal vaccines.

Antibody [immunoglobulin] DEFINITION Immunoglobulin (Ig) Immunoglobulin are glycoprotein molecules that are produced by plasma cells in response to an immunogen and protect the body against infection. CH : constant region of heavy chain VH : variable region of light chain CL : Constant region of light chain VL : variable region of light chain Fab: fragment antigen binding Fc : fragment crystallizable

A. Heavy and Light Chains All immunoglobulins have a four chain structure as their basic unit. They are composed of two identical light chains and two identical heavy chains . e.g. Ig G =λ chain

B. Variable (V) and Constant (C) Regions • 1. Light Chain - VL (110 amino acids) and CL (110 amino acids) • 2. Heavy Chain - (110 amino acids) and CH (330-440 amino acids)

C. Hinge Region This is the region at which the arms of the antibody molecule forms a Y. It is called the hinge region because there is some flexibility in the molecule at this point.

• D. Disulfide bonds • 1. Inter-chain disulfide bonds - The heavy and light chains and the two heavy chains are held together by inter-chain disulfide bonds and by non-covalent interactions The number of interchain disulfide bonds varies among different immunoglobulin molecules. • 2. Intra-chain disulfide bonds - Within each of the polypeptide chains there are also intra-chain disulfide bonds.

E. IMMUNOGLOBULIN FRAGMENTS: (A) Fab Digestion with papain breaks the immunoglobulin molecule in the hinge region before the H-H inter-chain disulfide bond. It has antigenic binding site.

B. Fc

Digestion with papain also produces a fragment that contains the remainder of the two heavy chains each containing a CH2 and CH3 domain. This fragment was called Fc because it was easily crystallized. They have plecental transfer site and complement binding site.

Clssification Of Antibody On The Basis Of Heavy chains 1. IgG IgG is the major Ig in serum - 75% of serum Ig is IgG. IgG is the major Ig in extra vascular spaces Location: Blood, lymph, intestine Half-life in serum: 23 days Complement Fixation: Yes Placental Transfer: Yes IgG is a good opsonin. The term opsonin is used to describe substances that enhance phagocytosis. Mol. Wt. : 1,50,000 e.g. Ig G =λ heavy chain

2.IgM Mol. Wt.: 9,00,000 e.g. µ heavy chain J Chain : IgM and IgA both contain J chain encoded by as separate gene called the joining chain which is disulphide bonded to the tailpieces, stabilising the multimer

Structure: Pentamer Percentage serum antibodies: 5-10% Location: Blood, lymph, B cell surface (monomer) Half-life in serum: 5 days Complement Fixation: Yes Placental Transfer: No Known Functions: First antibodies produced during an infection. Effective against microbes and agglutinating antigens. IgM is the third most common serum Ig.

3.IgA

Mol. wt. :3,20,000 e.g. α (Alpha) heavy chain

IgA is the 2nd most common serum Ig. Structure: Dimer Percentage serum antibodies: 10-15% Location: Secretions (tears, saliva, intestine, milk), blood and lymph. Half-life in serum: 6 days Complement Fixation: No Placental Transfer: No Known Functions: Localized protection of mucosal surfaces. Provides immunity to infant digestive tract.

4.IgD Structure: Monomer Percentage serum antibodies: 0.2% Location: B-cell surface, blood, and lymph Half-life in serum: 3 days Complement Fixation: No Placental Transfer: No Known Functions: In serum function is unknown. On B cell surface, initiate immune response. IgD is found in low levels in serum; its role in serum uncertain.

Ig D

Mol. Wt. : 1,80,000 e.g. δ (Delta) heavy chain

5.IgE Structure: Monomer Percentage serum antibodies: 0.002% Location: Bound to mast cells and basophils throughout body. Blood. Half-life in serum: 2 days Complement Fixation: No Placental Transfer: No Known Functions: Allergic reactions. Possibly lysis of worms. IgE is the least common serum Ig . IgE also plays a role in parasitic helminth diseases.

IgE

Mol. Wt. : 1,90,000 e.g. ε (Epsilon) Heavy chain

Monoclonal Antibodies Monoclonal Antibodies - identical antibodies produced in large quantities by an immortalized hybridoma cell line Technique developed by Köhler and Milstein in 1975, earned Nobel Prize in 1984 Benefits: identical antibodies available in unlimited quantity; easily purified Drawbacks: may not be useful for all techniques; limited number of species (mouse, rat, hamster, rabbit)

Monoclonal Antibody Production

•Process by which large quantities of antibodies (targeted against a particular antigen X) can be produced. •A mouse is immunized by injection of an antigen X to stimulate the production of antibodies targeted against X. The antibody forming cells are isolated from the mouse's spleen. •Monoclonal antibodies are produced by fusing single antibodyforming cells to tumor cells grown in culture. The resulting cell is called a hybridoma. •Each hybridoma produces relatively large quantities of identical antibody molecules. By allowing the hybridoma to multiply in culture, it is possible to produce a population of cells, each of which produces identical antibody molecules. These antibodies are called "monoclonal antibodies" because they are produced by the identical offspring of a single, cloned antibody producing cell.

Polyclonal antibody Polyclonal antibodies are antibodies that are derived from different B cell lines. They are a mixture of immunoglobulin molecules secreted against a specific antigen, each recognizing a different epitope These antibodies are typically produced by immunization of a suitable mammal, such as a mouse, rabbit or goat

An antigen is injected into the mammal. This induces the Blymphocytes to produce IgG immunoglobulins specific for the antigen. This polyclonal IgG is polyclonal purified from the mammal’s serum.

ANTIGENIC CLASSIFICATION OF ANTIBODY

There are three type: (1) ISOTYPE (2) ALLOTYPE (3) IDIOTYPE (1) ISOTYPE “Antigenically different immunoglobulin variants that are common in all individual of given biological species are called as isotype.” e.g. Ig G that is subdivided into subclasses IgG1,IgG2,IgG3,IgG4. (2) ALLOTYPE “Determinants on immunoglobulin molecules that differ among individual of same species have inherited different allels.” (3) IDIOTYPE “If any antigen determinants or an epitope is localized in a variable domain of the immunoglobulin chain is called as Idiotype.”

Major Histocompatibility Complex (MHC)  The MHC is a closely linked complex of genes that govern production of the major histocompatibility  In humans, MHC resides on the short arm of chromosome 6  Three genes (HLA-A, HLA-B, HLA-C) code for  the class I MHC proteins  Several HLA-D loci determine the class II MHC proteins i.e. DP, DQ and DR  HLA genes are very diverse (polymorphic)  i.e. there are many alleles of the class I and II genes

Major Histocompatibility Complex (MHC) 

Between the class I and class II gene loci, there is a third locus (Class III)

 This locus contains genes encoding tumor necrosis factor, lymphotoxin and two complement components (C2 and C4)  Class III antigens do not participate in MHC restriction or graft rejection

Major Histocompatibility Complex (MHC)  Class I MHC antigens are : HLA-A, HLA-B and HLA-C  These antigens are glycoproteins found on surfaces of all nucleotide human cells and on platelets  HLA-A contains 24 different antigenic specificities,  HLA-B contains 52 and HLA-C contains 11  Class I MHC antigens are involved of MHC restriction of cell mediated cytotoxicity

Major Histocompatibility Complex (MHC)  Endogenously processed cytosolic peptides in virus infected cells or tumor cells are transported to the surface of the cells  They bind to MHC I molecules to be recognized by cytotoxic T-cells which then kill these cells  In other words; T-cells are only activated when they recognize both antigen and class I MHC molecules in association

Major Histocompatibility Complex (MHC) • Class II antigens are: HLA-DP, HLA-DQ, HLADR antigens • These antigens are glycoproteins found on the surface of macrophages, B-cells, Dentritic cells, langerhans cells of skin and activated T cells • HLA-DP contain 6 different antigenic specificities, HLA-DQ contains 9 and HLA-DR contains 20 •

Class I MHC and Class II MHC MHC Class I

MHC Class II

HLA-A, HLA-B, HLA-C

HLA-DP, HLA-DQ, HLA-DR

Found on

All nucleated somatic cells

Macrophages, B-cells, Dentritic cells, langerhans cells of skin and activated T cells

Recognized by

CD8 TC cells

CD4 TH cells

Nomenclature

Functions

Presentation of Ag to TC Presentation of Ag to TH cells leading to elimination cells which secrete of tumor or infected host cytokines cell

Types of grafts • 1) Autografts : • The transfer of an individual’s own tissues • from place to place • e.g. Skin grafts (regularly accepted) • 2) Isografts : • Transfer of tissues between genetically • identical persons • e.g. Identical twins ( accepted permanently

3) Allografts (homograft): - Transfer of a graft between genetically different members of same species e.g from one human to another - Rejection occur if donor and recipient are not matched 4) Xenograft (heterograft): - Transfer of tissues between different species - Always rejected