Agntigen micriobioly immunity immunity system

antigen

1. IMMUNOLOGY 1

2. ANTIGEN Substance which when introduced parenterally into the body stimulates the production of antibodies with which it reacts specifically and in an observable manner. Substance which when introduced parenterally into the body stimulates the production of humoral or cellular immune response with which it reacts specifically in an observable manner. 2

3. More appropriately: Antigen is defined as any substance that satisfies 2 distinct immunologic properties- immunogenicity and antigenicity . Immunogenicity: It is the ability of an antigen to induce immune response in the body (both humoral and/or cell mediated). These are called Immunogen . Antigenicity (Immunological reactivity): It is the ability of an antigen to combine specifically with the final products of humoral and cell mediated immune responses. All molecules having immunogenicity also show antigenicity , but the reverse is not true. Eg : Haptens . 3

4. Types of Antigen Based on Immunogenicity and immunological reactivity: Complete Antigen Incomplete Antigen/ Haptens : Complex haptens Simple haptens 4

5. COMPLETE ANTIGEN These are substances which can induce antibody formation by themselves and can react specifically with these antibodies and produce observable reaction. 5

6. HAPTENS Haptein : Grasp These are substances which are unable to induce antibody formation on its own but can become immunogenic when linked to larger protein molecules, called as carrier proteins . Eg : Penicillin, Pollens, Urushiol etc.. Complex haptens : can combine with specific antibodies to form precipitates because of having multiple antibody combining sites (Polyvalent). Simple haptens : Combines with specific antibodies but no precipitate is produced due to being univalent. 6

7. Antigenic Determinant A small area of chemical grouping on the antigen molecule that determines specific immune response and reacts specifically with antibody. Also known as an ‘ epitope ’ . Paratope : Combining site on the antibody molecule for epitope . 7

8. Exogenous Antigens 1. Bacterial antigens : a) - Antigens related to bacterial cells - Somatic antigen (O): part of cell wall - Capsular antigen (K): usually polysaccharide - Flagellar Ag (H) : a protein made of flagellin - Fimbrial Ag (F): surface antigens in fimbriated bacilli b) - Antigen secreted by bacteria: - Exotoxins - Enzymes 2. Viral antigens : a) protein coat -viral antigens b) nucleoproteins as in influenza Types of Antigens 8

9. Endogenous antigens Human tissue antigens: a) Blood group antigens: A, B and Rh antigens b) Histocompatibility antigens: Glycoprotein molecules on all nucleotide cells: - Major histocompatibility complex antigens (MHC) - Human leucocyte antigen (HLA) Types Of Antigens 9

10. Types of Antigen Based on functioning: Immunogen : The antigen that induce specific immune response. Allergen : The antigen that causes allergic reaction (production of IgE ). E.g.: toxins, pollens… Vaccines : An antigen that induces a protective immune response. 10

11. Factors of Antigenicity : These factors are the properties that makes a substance antigenic. Exact basis is still unclear. Foreignness Size Chemical nature Susceptibility to tissue enzymes 5. Species specificity 6. Isospecificity 7. Autospecificity 8. Organ specificity 9. Heterophile specificity. 11

12. Factors of Antigenicity : 1. Foreignness: Should be non-self. More foreign, more powerful the antigen. Immune system normally does not mount an immune response against its own antigens. This tolerance is due to continuous contact with them during developmental stages. 2. Size: Larger molecules are highly antigenic whereas low molecular weight (less than 10,000Da) are either non-antigenic or weakly antigenic. But may be rendered antigenic by adsorbing these on inert particles like bentonite or kaolin. 12

13. Factors of Antigenicity : Chemical nature: Most naturally occurring antigens are either proteins or polysaccharides. Lipids and nucleic acids are less antigenic on their own but do so when combined with proteins. Protein > Carbohydrates > Lipids 4. Susceptibility to Tissue Enzymes: Substances which can be metabolized and are susceptible to the action of tissue enzymes behave as antigens. Eg : Bovine serum albumin has more than 18 determinants although only 6 of these are exposed in the intact molecule. 13

14. Factors of Antigenicity : 5. Species Specificity: Tissue of all individuals in a species possess common species specific Ag. Forensic applications for identification of species of blood. 6. Isospecificity : Depends on isoantigens which may be found in some but not all members of a species. Eg : Blood group antigens . Histocompatibility antigens are associated with the plasma membrane of tissue cells. In tissue transplant. Human leucocyte antigen (HLA) - homograft rejection. 14

15. Factors of Antigenicity : 7. Autospecificity : Exceptions are there in self antigens which are generally non-antigenic. Eg : Lens protein, sperm (Sequestered antigen) . Autoimmune diseases has same mechanism in pathogenesis. 8. Organ Specificity: These organ specific antigens are confined to a particular organ. Some organ like brain, kidney and lens protein of one species share specificity with that of another species. Eg : Brain specific antigens are shared by man and sheep, so antirabies vaccines (sheep brain vaccines) may cause neuroparalytic complications in human. 15

16. Factors of Antigenicity : Heterophile Specificity: Same or closely related antigens present in different tissues of more than one species. Antibodies to these antigens produced by one species cross react with antigens of other species. Eg : Blood group B Ag & E. coli 086 Paul Bunnel Test- Epstein Barr virus » Ab » agglutinates sheep erythorcytes . Weil Felix reaction- Rickettsiae (typhus fever) » Ab » agglutinates strains of Proteus species 16

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18. SUPERANTIGENS ( SAgs ) Molecules that can interact with antigen presenting cells and T- lymphocytes in a non-specific manner. SAgs are produced by some pathogenic viruses and bacteria most likely as a defense mechanism against the immune system. Causes non-specific activation of T-cells causing massive cytokines release. E.g.: Streptococcal & Staphylococcal enterotoxins ; exfoliative toxin and some viral proteins such as of Ebola, HIV, Epstein-Barr virus. 18