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Antigen concept made easy

The document is a comprehensive overview of antigens, covering their definitions, chemical nature, types, and mechanisms of action. It distinguishes between antigens and immunogens, describes various antigen classifications (e.g., based on origin and immune response), and explains concepts such as haptens and super antigens. Additionally, it discusses methods of antigen detection and their applications in laboratory diagnostics.

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Page 2 1. DEFINITION. 2. CHEMICAL NATURE OF ANTIGEN. 3. TYPES OF ANTIGEN. 4. HAPTENS. 5. SUPER ANTIGEN. 6. DETERMINANTS OF ANTIGENICITY. 7. ADJUVANTS. 8. EPITOPES. 9 . USES. 10. ANTIGEN DETECTION METHOD.
Page 3 ANTIGENS Definition:- Any substance when introduced into the body stimulate the production of antibody and react with the specific antibody and antigen receptors present on lymphocytes are called antigen. Immunogens:- Are the molecules which induce an immune response. All antigens are immunogens, but all immunogens are not antigen.
Page 4 CHEMICAL NATURE OF ANTIGEN • Most naturally occurring antigens are proteins , polysaccharides followed by lipids and nucleic acids. • All proteins are not antigenic in nature.
Page 5 TYPES OF ANTIGEN 1. Based on the ability to carry immunogenicity and immunological reaction: a) Complete antigen. b) Hapten. 2. Based on origin: I. a) Microbial antigen. II. a) Exogenous b)Tissue antigen. b) Endogenous
Page 6 3. Based on immune response: a) T-cell dependent antigen. b) T-cell independent antigen(TI) Other antigens: Super antigens. Histocompatibility antigens or transplantation antigens. Tumor antigens.
Page 7 a) Complex antigen or complete antigen: A complete antigen is able to induce antibody formation and produce a specific and observable reaction with the antibody produced. It has mainly 2 properties: a) Immunogenicity b) Immunological reaction
Page 8 HAPTENS These are low molecular weight substance that is not immunogenic by itself, but can react with antibody produced by it. Haptens are not immunogenic because,  Failure of hapten to activate helper T cells.  Haptens are univalent, hence cannot activate B-cells .  Low molecular weight.
Page 9 How hapten can become immunogen? Carrier protein + Hapten B cell B cell Helper T cell B cell Plasma cell B cell B cell Helper T cell B cell Plasma cell IgM IL-4, IL-5 TCR Epitope Class II MHC protein Antibodies specific for hapten
Page 10 Types of haptens 1. Complex hapten – One that can participate in precipitation reaction and are usually polyvalent in nature. 2. Simple hapten – Hapten which do not participate in precipitation reaction is called as simple hapten.
Page 11 ADJUVANTS Adjuvants are the substance that when mixed with an antigen and injected with it, boost the immunogenicity of the antigen. Adjuvants increase both strength and duration of immune response. E.g. Aluminium potassium sulphate(alum) Freund’s water-in-oil
Page 12 Functions of adjuvant i. Boost immune response when antigen has low immunogenecity and when small amount of antigen is available ii. enhance co-stimulatoty signals iii. Prolong retention of immunogen iv. Increase the effective size of the immunogen v. Increase local inflammation by stimulating local influx of macrophages or immune cells to injected sites
Page 13 2) Based on origin: I. Microbial antigen: These are structural antigens. e.g., Flagellar H-antigen, Fimbrilar antigen. Tissue antigen: e.g, Blood group antigen, Transplantation antigen. II Exogenous antigen: Antigen which are present outside the cell. Endogenous antigen: Antigen of self immune system.
Page 14 3. Based on immune response. a) T cell dependent antigen (TD). b) T cell independent antigen (TI). TI type I TI type II
Page 15 a) T-cell dependent antigen(TD): Antigen that require help of T cell to activate B cell for antibody production is called T dependent antigen. e.g. Erythrocytes, serum proteins.  Induce immunoglobulin isotypes – IgM, IgG, IgA,and IgE.  Show immunological memory.  Require preliminary processing.  Rapidly metabolized in the body.
Page 16 b) T cell independent antigen(TI): Antigen directly stimulates antibody production without the help of T-cell are called TI antigen. e.g. Pneumococcal capsular polysaccharides. Flagellar protein - flagellin. There are 2 subtypes: i) TI type I:  These are antigens of microbial origin.  Can activate both mature and immature B cell. e.g. Lipopolysaccharides.
Page 17 ii) TI type II: These are polysaccharides that can activate only mature B cell.  Consisting of limited number of epitopes.  Antibody response is usually limited to IgM and IgG3.  Do not show immunological memory.  These are metabolized slowly and remain in the body for long periods.
Page 18 SUPERANTIGEN Superantigens are the antigens which polyclonally activate a large fraction or number of the T cells (up to 25%) . E.g. a) Staphylococcal enterotoxins (food poisoning). b) Streptococcal pyogenic exotoxins. c) Mouse mammary tumor virus.
Page 19 Histocompatibility antigen or Transplantation antigen: Antigen that participate in graft rejection are called transplantation or histocompatibility antigen. e.g. Blood group antigen are important in transplantation. Tumor Antigens: Tumor antigens are antigen that are present in malignant cells but are absent in corresponding normal cells of the host. e.g. 1. Alpha fetoprotein in hepatomas. 2.Carcinoembryonic antigen found in colonic cancer
Page 20 Determinants of antigenicity or immunogenecity: a. Size. b. Chemical structures. c. Foreignness. d. Susceptibility to tissue enzyme. e. Antigenic specificity. f . Species specificity. g . Isospecifity. h . Autospecificity. i . Heterophile specificity
Page 21 a) Size: A substance or molecule should have optimum size to induce immunogen (10,000 D). Smaller molecules may either be weakly or non immunogens. E.g. Heamocyanin - large molecule and highly immunogen. Haptens - low molecular weight molecule and are non immunogenic .
Page 22 b)Chemical structure: Proteins are highly immunogenic followed by polysaccharide, lipids and nucleic acid. Note: Gelatin is not immunogenic because of increased tyrosin content. c)Foreignness: Antigens which are foreign to the individual (non self) induce an immune response. Self antigen are usually not immunogenic because they do not come in contact with immune system during the developmental stage. Breakdown of this mechanism will result in autoimmune disease. Self antigen E.g . Eye lens antigen.
Page 23 d) Susceptibility to tissue enzymes: Substances which can be metabolised and are susceptible to the action of tissue enzymes behave as a good antigen. Substance that are not susceptible to tissue enzyme are not immunogenic. e.g. Polystyrene latex. Substance that cannot be metabolised in the body e.g. D- aminoacid. e) Anitigenic specificity: Depends on position of antigenic determinant group in the antigen molecule.
Page 24 f) Species specificity: These antigens are present in all members or individual of a species. Species specific antigen has got phylogenetic relationship used to trace evolutionary relationship and in forensic application for the identification of species of blood and seminal fluid. g) Isospecificity: It depends on isoantigen present on some but not all the members of species. They are genetically determined. e.g. Blood group antigen, transplantation antigen, HLA antigen.
Page 25 h) Autospecificity: Self antigen are usually non antigenic but there are exception – Lens protein and sperm. These are not recognized as self antigen because they do not come in contact with immune system, therefore it is not recognised. when these are released into tissue following injury antibodies are produced against them resulting in autoimmune disease. i) Heterophile specificity: Same or closely related antigen present in different tissues of more than one species are known as heterophile antigen. E.g, 1.Forssman antigen- absent in rabbit. 2. Antigen used in Weil-Felix reaction. 3. Antigen used in Paul-Bunnel reaction.
Page 26 J) Organ specificity: Organ specific antigens are confined to a particular organ. Some organs like Brain, kidney and lens protein of one species share specificity with that of another species. E.g. Brain specific antigen are shared by man and sheep.
Page 27 EPITOPES (ANTIGENIC DETERMINANT) Immunologically active region of an immunogen that binds to antigen-specific membrane receptors on lymphocytes or secreted antibodies. Structure: Size: Very small, consisting of four or five aminoacid or monosaccharides residues. Conformation: Linear Non linear. Site: Internal(with in antigen) external(on the antigen)
Page 28 Uses: Laboratory diagnosis of infectious disease by antigen detection. Diagnosis of non infectious diseases. e.g. Autoimmune disease. In epidemiology of infectious diseases. Detection of enzyme or hormone (hCG) Used in vaccine preparation.
Page 29 Antigen detection methods: 1.Enzyme Immuno Assay for detecting antigen in a) Bacterial antigens:  Capsular antigen of Streptococcus pneumoniae.  Haemophilus influenzae type b. b) In virology:  Rota virus in stool filtrates.  Respiratory syncytial virus in case of lower respiratory tract specimen.
Page 30 c) In Parasitology:  E. histolytica dispar  Giardia lamblia  Cryptosporidium parvum. d) In Mycology:  Histoplasma and Cryptococcus antigen test.
Page 31 2. Immunofluorescence technique. 3.Agglutination test. 4.Immunodiffusion test. 5.Complement fixation test
Page 32 REFERENCES :- 1. Review of Medical Microbiology and Immunology by Warren Levinson, 10th edition. 2. Text Book of Microbiology by Ananthanarayan and Paniker, 7th edition. 3. Immunology, International edition by Ivan Roitt, David B Roth, Jonathan Brastaff, David Male, 7th edition. 4. Immunology by Kuby, 6th edition. 5. Encylopedia of immunology.
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