Antigens can be classified into three types:An antigen may be exogenous (from outside the host immune system), endogenous (produced by intracellular bacteria and viruses replicating inside the host cell), or autoantigens (produced by the host). A sensitized immune system will mount an attack on autoantigens, resulting in an autoimmune disease. In normal situations, autoantigens do not elicit an immune response. Allergies are a common term for autoantigens present in the body.