ABSTRACT
An antigen was originally considered simply to be any substance capable of inducing the generation of antibody. In fact, the word antigen was coined from the words “antibody” and “generator.” Cross-reactivity may thus be the result of the presence of common molecules in complex antigens of related species. In complex antigens, such as preparations of microbes, cross-reactivity may result from the presence of a variety of molecules in the preparation some of which are shared. The binding site for superantigens, unlike a processed antigen fragment, is outside the antigen-binding site of the MHC. Superantigens are a group of molecules that function very differently from traditional immunogens. An antigenic determinant is defined as a region of an antigenic molecule that fits into the combining site of an antibody. It comprises only a small part of the invading pathogen or molecuLe to which the immune system of the host has responded.
