Endogenous antigens are antigens that have been generated within the body. They include xenogenic (heterologus), autologus, idiotypic or allogenic antigens and autoantigens.
Xenogeneic Antigen:
These are also known as “hetero antigens”. It is an antigen common to members of one species but not to members of other species. Generally immune system does not produce any antibodies against self antigens but the antibodies produced against foreign agents some times react with certain surface molecules of their own cells due to their similarity with the foreign antigen.
The surface molecules or antigens that are attacked by self antibodies are known as xenogenic antigens. The cardiolipin antigen, forssman antigen present on cells is some of the xenogenic antigens that show similarity with antigens present on pneumococci and salmonella bacteria.
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Cardiolipin antigen and Forssman antigen: Heterogenetic antigens occurring in various unrelated species, mainly in the organs but not in the erythrocytes (guinea pig, horse), but sometimes only in the erythrocytes (eg sheep), and occasionally in both (eg chicken).
Autologous antigens: –
Certain auto antigens that were not exposed to immune system during development are recognized as foreign antigens. For example the antigens present on the sperm are not recognized by the immune system as its own, because the development of sperms takes place during adolescence and immune cells never come across the developing sperm cells.
In the same way proteins of cornea are also identified as foreign antigens (Refer Chapter 15 for autoimmunity)
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Allogenic antigens:
Antigens such as blood group antigens, tissue antigens are under the control of genes.
Since the genes controlling the antigens are polymorphic in nature the antigens also express polymorphism and are specific for each individual. Exact matching of these antigens between two individual is not possible, due to different combinations of genes.
Identical twins alone show exact matching because of genetical similarity. Allogenic antigens are responsible for blood transfusion reactions and graft rejection reactions between individuals of same species.