ANTIGEN
Babitha Elias
DEFINITION:
Antigen - substance which reacts with
specific antibody.
Immunogen - substance that can induce
immune response.
TYPES OF ANTIGEN
 Complete Ag
 Haptens / incomplete Ag
COMPLETE ANTIGEN
 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.
 Haptens can become immunogenic when
linked to a carrier protein.
Haptens – 2 types
Complex haptens:
 Can combine with specific antibodies to
form precipitate.

Eg: capsular polysaccharide of Pneumococci
Simple haptens:
 Can combine with specific antibodies but
no precipitate is formed.
 They are monovalent.
Antigenic determinant / Epitope
 Smallest unit of antigenicity.
 Each determinant induces a specific
antibody formation.
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FACTORS OF ANTIGENICITY
Foreignness
Size
Chemical nature
Susceptibility to tissue enzymes
Antigenic specificity
Species specificity
Isospecificity
Autospecificity
Organ specificity
Heterophile specificity
FOREIGNNESS


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.
SIZE


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Larger molecules (> 10,000 Da) are highly
antigenic.
Low mol.weight substances are either non
antigenic or weakly antigenic.
Haptens are low mol.weight – non
immunogenic.
CHEMICAL NATURE
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Most antigens are proteins, but not all
proteins are immunogenic.
Exception: Gelatin
Carbohydrates, lipids, nucleic acids are
less antigenic.
SUSCEPTIBILITY TO TISSUE ENZYMES

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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.

Eg: polystyrene latex
ANTIGENIC SPECIFICITY
 Depends on antigenic determinants.
 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
 Helps in


Tracing of evolutionary relationship
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.
 They are genetically determined.
 Eg: Blood group antigens,
Histocompatibility Ags.
AUTOSPECIFICITY
 Self antigens are non antigenic, with some
exceptions.
 Lens protein & Sperm
ORGAN SPECIFICITY
 Organ specific Ags are confined to a
particular organ.
 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.
 Eg:
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Forssman Ag
Weil- Felix reaction
Paul – Bunnell test
Forssman Ag
 Lipoprotein – polysaccharide complex
present in man, animals, birds & bacteria.
Weil-Felix reaction
 Used for the diagnosis of Typhus fever.
 Serum of the patient agglutinates strains
of Proteus spp.
 Sharing of heterophile Ag on Rickettsiae
and Proteus strains
Paul- bunnel test
 For diagnosis of Infectious mononucleosis.
 Heterophile Abs agglutinate sheep
erythrocyte.
SUPER ANTIGENS
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Molecules that can interact with antigen
presenting cells and T-lymphocytes in a
non specific manner.
Eg: Staphylococcal enterotoxin, Exfoliative
toxin.
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