1. ENDOGENOUS ANTIGENS
Endogenous antigens are naturally occurring antigenic substances found within an animal or human and which are unique to every individual. They are generally responsible for tissue rejection experienced during the transplanting of cells, organs or tissues from one host (donor) to another host (recipient). Typical examples of endogenous antigens are allogenic antigens. Allogenic antigens are largely controlled by antigenic determinants that differentiate one individual of a species from another. These allogenic antigens are specific cell markers found on the surfaces of cells and tissues of every individual and animal. They are often seen as antigens by the immune system when they move from one person to another in scenarios of blood transfusion or tissue transplant.
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2. EXOGENOUS ANTIGENS
Exogenous antigens are foreign molecules or substances that are external to the body of a host. They include pathogenic microorganisms and their products, proteins of high molecular weight and polysaccharides as well. Exogenous antigens are responsible for a handful of infectious diseases in man and animals, and they elicit both specific and non-specific immunological attack upon entry into the body of a host. It is these antigens that are recognized by the host’s immune system – in which case, a specific immunological response (e.g., production of a specific antibody) is directed against it. Endogenous antigens are foreign to the host’s body; and when recognized by the immune system upon entry, specific immunological molecules including antibodies are released to stop their further spread or harm in the individual.
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