ANTIGENANTIGEN

Babitha EliasBabitha Elias

DEFINITION:DEFINITION:

Antigen Antigen -- substance which reacts with substance which reacts with specific antibody. specific antibody.

ImmunogenImmunogen - substance that can - substance that can induce immune response.induce immune response.

TYPES OF ANTIGENTYPES OF ANTIGEN Complete AgComplete Ag Haptens / incomplete AgHaptens / incomplete Ag

COMPLETE ANTIGENCOMPLETE ANTIGEN Substances which can induce Substances which can induce

antibody formation by themselves antibody formation by themselves and can act specifically with these and can act specifically with these antibodies.antibodies.

HAPTENSHAPTENS Substances unable to induce Substances unable to induce

antibody formation on its own.antibody formation on its own. Haptens can become immunogenic Haptens can become immunogenic

when linked to a carrier protein.when linked to a carrier protein.

Haptens – 2 typesHaptens – 2 types

Complex haptens:Complex haptens: Can combine with specific antibodies Can combine with specific antibodies

to form precipitate.to form precipitate. Eg: capsular polysaccharide of Eg: capsular polysaccharide of

PneumococciPneumococci

Simple haptens:Simple haptens: Can combine with specific antibodies Can combine with specific antibodies

but no precipitate is formed.but no precipitate is formed. They are monovalent.They are monovalent.

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

antibody formation.antibody formation.

FACTORS OF ANTIGENICITYFACTORS OF ANTIGENICITY1.1. ForeignnessForeignness

2.2. SizeSize

3.3. Chemical natureChemical nature

4.4. Susceptibility to tissue enzymesSusceptibility to tissue enzymes

5.5. Antigenic specificityAntigenic specificity

6.6. Species specificitySpecies specificity

7.7. IsospecificityIsospecificity

8.8. AutospecificityAutospecificity

9.9. Organ specificityOrgan specificity

10.10. Heterophile specificityHeterophile specificity

FOREIGNNESSFOREIGNNESS

An antigen must be non-self/ foreign An antigen must be non-self/ foreign to induce an immune response.to induce an immune response.

Immune system does not mount an Immune system does not mount an immune response to the self immune response to the self antigens – self tolerance.antigens – self tolerance.

SIZE SIZE

Larger molecules (> 10,000 Da) are Larger molecules (> 10,000 Da) are highly antigenic.highly antigenic.

Low mol.weight substances are Low mol.weight substances are either non antigenic or weakly either non antigenic or weakly antigenic.antigenic.

Haptens are low mol.weight – non Haptens are low mol.weight – non immunogenic.immunogenic.

CHEMICAL NATURECHEMICAL NATURE

Most antigens are proteins, but not Most antigens are proteins, but not all proteins are immunogenic.all proteins are immunogenic.

Exception: GelatinException: Gelatin Carbohydrates, lipids, nucleic acids Carbohydrates, lipids, nucleic acids

are less antigenic.are less antigenic.

SUSCEPTIBILITY TO TISSUE SUSCEPTIBILITY TO TISSUE ENZYMESENZYMES

Substances which can be Substances which can be metabolized and are susceptible to metabolized and are susceptible to the action of tissue enzymes act as the action of tissue enzymes act as good immunogens.good immunogens.

Substances unsusceptible to tissue Substances unsusceptible to tissue enzymes are not antigenic.enzymes are not antigenic. Eg: polystyrene latexEg: polystyrene latex

ANTIGENIC SPECIFICITYANTIGENIC SPECIFICITY Depends on antigenic determinants.Depends on antigenic determinants. The position of the antigenic group in The position of the antigenic group in

the Ag molecule is important for the Ag molecule is important for specificity.specificity.

SPECIES SPECIFICITYSPECIES SPECIFICITY Tissues of all individuals in a species Tissues of all individuals in a species

possess species specific Agspossess species specific Ags Helps in Helps in

Tracing of evolutionary relationshipTracing of evolutionary relationship Forensic application in the identification of Forensic application in the identification of

species of blood & seminal fluid.species of blood & seminal fluid.

ISOSPECIFICITYISOSPECIFICITY Depends on isoantigens, which may Depends on isoantigens, which may

be found in some but not all be found in some but not all members of a species.members of a species.

They are genetically determined.They are genetically determined. Eg: Blood group antigens, Eg: Blood group antigens,

Histocompatibility Ags.Histocompatibility Ags.

AUTOSPECIFICITYAUTOSPECIFICITY Self antigens are non antigenic, with Self antigens are non antigenic, with

some exceptions.some exceptions. Lens protein & SpermLens protein & Sperm

ORGAN SPECIFICITYORGAN SPECIFICITY Organ specific Ags are confined to a Organ specific Ags are confined to a

particular organ.particular organ. Organs like brain, kidney, lens Organs like brain, kidney, lens

protein share specificity with that of protein share specificity with that of another species.another species.

HETEROPHILE SPECIFICITYHETEROPHILE SPECIFICITY

Heterophile AgHeterophile Ag :– same or closely :– same or closely related Ags present in different related Ags present in different tissues of more than one species.tissues of more than one species.

Abs to these Ags produced by one Abs to these Ags produced by one species cross react with Ags of other species cross react with Ags of other species.species.

Eg:Eg: Forssman AgForssman Ag Weil- Felix reactionWeil- Felix reaction Paul – Bunnell testPaul – Bunnell test

Forssman AgForssman Ag Lipoprotein – polysaccharide complex Lipoprotein – polysaccharide complex

present in man, animals, birds & present in man, animals, birds & bacteria.bacteria.

Weil-Felix reactionWeil-Felix reaction Used for the diagnosis of Typhus fever.Used for the diagnosis of Typhus fever. Serum of the patient agglutinates strains Serum of the patient agglutinates strains

of Proteus spp.of Proteus spp. Sharing of heterophile Ag on Rickettsiae Sharing of heterophile Ag on Rickettsiae

and Proteus strainsand Proteus strains

Paul- bunnel testPaul- bunnel test For diagnosis of Infectious For diagnosis of Infectious

mononucleosis.mononucleosis. Heterophile Abs agglutinate sheep Heterophile Abs agglutinate sheep

erythrocyte.erythrocyte.

SUPER ANTIGENSSUPER ANTIGENS

Molecules that can interact with Molecules that can interact with antigen presenting cells and T-antigen presenting cells and T-lymphocytes in a non specific lymphocytes in a non specific manner.manner.

Eg: Staphylococcal enterotoxin, Eg: Staphylococcal enterotoxin, Exfoliative toxin.Exfoliative toxin.

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