Understanding The Immune System

Idiotypes Every antibody's unique structure can themselves act as an antigen. Known as idiotypes, theses structures can trigger a complementary antibody, or antiidiotype. The antiidiotype can sometimes be substituted for the original antigen.

A Web of Idiotypes The unique and characteristic pocket on an antibody that recognizes a specific antigen-its variable region-can itself act as an antigen. More precisely the variable region contains a number of antigen-like segments, and these are known collectively as an idiotype. Like any other antigen, an idiotype can trigger complementary antibody. This second round antibody is known as an antiidiotype. An antiidiotype, in turn, can trigger an antiantiidiotype. Like a series of mirrored reflections, the process can go on and on. Interactions between idiotypes and antiidiotypes, it has been proposed, constitute a mechanism whereby the immune system regulates itself. According to the "network theory" not only antibodies but B cells and T cells carry-in their unique antigen-receptors-idiotypes. The B cells and T cells that proliferate in response to a certain antigen carry a complementary idiotype. Antiidiotype B cells secrete anti idiotype antibodies, which may neutralize the original idiotypes (antibodies), or bind to idiotypes on regulatory T cells. Alternatively, antiidiotypes may trigger antiantlidiotypes, creating a spiraling response within the network-turning on, amplifying, and shutting down immune responses. The concept of the idiotype is being put to practical use today in the development of experimental antigen-free vaccines (see Vaccines through Biotechnology).