AntigenAntigen means foreign particles (cells, soluble substances) and diversity in size and chemical composition. The most common antigens are exoantigens that are often microorganisms and their products. Andoantigens (endogenous antigens) are antigens that come from their own organism.
Antigens may be proteins on the surface of bacteria, viruses and other infectious organisms or on foreign transplanted cells. An antigen may also be a more complex chemical that does not occur naturally in the body.