ANTIGEN Babitha Elias

DEFINITION: Antigen - substance which reacts with specific antibody. Immunogen - substance that can induce immune response. TYPES OF ANTIGEN Complete Ag Complete Ag Haptens / incomplete Ag Haptens / incomplete Ag

COMPLETE ANTIGEN Substances which can induce antibody formation by themselves and can act specifically with these antibodies. Substances which can induce antibody formation by themselves and can act specifically with these antibodies.HAPTENS Substances unable to induce antibody formation on its own. Substances unable to induce antibody formation on its own. Haptens can become immunogenic when linked to a carrier protein. Haptens can become immunogenic when linked to a carrier protein.

Haptens – 2 types Complex haptens: Can combine with specific antibodies to form precipitate. Can combine with specific antibodies to form precipitate. Eg: capsular polysaccharide of Pneumococci Eg: capsular polysaccharide of Pneumococci Simple haptens: Can combine with specific antibodies but no precipitate is formed. Can combine with specific antibodies but no precipitate is formed. They are monovalent. They are monovalent.

Antigenic determinant / Epitope Smallest unit of antigenicity. Smallest unit of antigenicity. Each determinant induces a specific antibody formation. Each determinant induces a specific antibody formation.

FACTORS OF ANTIGENICITY 1. Foreignness 2. Size 3. Chemical nature 4. Susceptibility to tissue enzymes 5. Antigenic specificity 6. Species specificity 7. Isospecificity 8. Autospecificity 9. Organ specificity 10. Heterophile specificity

FOREIGNNESS An antigen must be non-self/ foreign to induce an immune response. An antigen must be non-self/ foreign to induce an immune response. Immune system does not mount an immune response to the self antigens – self tolerance. Immune system does not mount an immune response to the self antigens – self tolerance.

SIZE Larger molecules (> 10,000 Da) are highly antigenic. Larger molecules (> 10,000 Da) are highly antigenic. Low mol.weight substances are either non antigenic or weakly antigenic. Low mol.weight substances are either non antigenic or weakly antigenic. Haptens are low mol.weight – non immunogenic. Haptens are low mol.weight – non immunogenic.

CHEMICAL NATURE Most antigens are proteins, but not all proteins are immunogenic. Most antigens are proteins, but not all proteins are immunogenic. Exception: Gelatin Exception: Gelatin Carbohydrates, lipids, nucleic acids are less antigenic. Carbohydrates, lipids, nucleic acids are less antigenic.

SUSCEPTIBILITY TO TISSUE ENZYMES Substances which can be metabolized and are susceptible to the action of tissue enzymes act as good immunogens. Substances which can be metabolized and are susceptible to the action of tissue enzymes act as good immunogens. Substances unsusceptible to tissue enzymes are not antigenic. Substances unsusceptible to tissue enzymes are not antigenic. Eg: polystyrene latex Eg: polystyrene latex

ANTIGENIC SPECIFICITY Depends on antigenic determinants. Depends on antigenic determinants. The position of the antigenic group in the Ag molecule is important for specificity. The position of the antigenic group in the Ag molecule is important for specificity. SPECIES SPECIFICITY Tissues of all individuals in a species possess species specific Ags Tissues of all individuals in a species possess species specific Ags Helps in Helps in Tracing of evolutionary relationship Tracing of evolutionary relationship Forensic application in the identification of species of blood & seminal fluid. Forensic application in the identification of species of blood & seminal fluid.

ISOSPECIFICITY Depends on isoantigens, which may be found in some but not all members of a species. Depends on isoantigens, which may be found in some but not all members of a species. They are genetically determined. They are genetically determined. Eg: Blood group antigens, Histocompatibility Ags. Eg: Blood group antigens, Histocompatibility Ags.

AUTOSPECIFICITY Self antigens are non antigenic, with some exceptions. Self antigens are non antigenic, with some exceptions. Lens protein & Sperm Lens protein & Sperm ORGAN SPECIFICITY Organ specific Ags are confined to a particular organ. Organ specific Ags are confined to a particular organ. Organs like brain, kidney, lens protein share specificity with that of another species. Organs like brain, kidney, lens protein share specificity with that of another species.

HETEROPHILE SPECIFICITY Heterophile Ag :– same or closely related Ags present in different tissues of more than one species. Abs to these Ags produced by one species cross react with Ags of other species. Abs to these Ags produced by one species cross react with Ags of other species. Eg: Eg: Forssman Ag Forssman Ag Weil- Felix reaction Weil- Felix reaction Paul – Bunnell test Paul – Bunnell test

Forssman Ag Lipoprotein – polysaccharide complex present in man, animals, birds & bacteria. Lipoprotein – polysaccharide complex present in man, animals, birds & bacteria. Weil-Felix reaction Used for the diagnosis of Typhus fever. Used for the diagnosis of Typhus fever. Serum of the patient agglutinates strains of Proteus spp. Serum of the patient agglutinates strains of Proteus spp. Sharing of heterophile Ag on Rickettsiae and Proteus strains Sharing of heterophile Ag on Rickettsiae and Proteus strains

Paul- bunnel test For diagnosis of Infectious mononucleosis. For diagnosis of Infectious mononucleosis. Heterophile Abs agglutinate sheep erythrocyte. Heterophile Abs agglutinate sheep erythrocyte.

SUPER ANTIGENS Molecules that can interact with antigen presenting cells and T-lymphocytes in a non specific manner. Molecules that can interact with antigen presenting cells and T-lymphocytes in a non specific manner. Eg: Staphylococcal enterotoxin, Exfoliative toxin. Eg: Staphylococcal enterotoxin, Exfoliative toxin.

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