[Molecular genetic studies on the immune system: an approach to a clinical application].

It is the function of the immune system to recognize and neutralize numerous antigens invading animals. The more diverse the antigen recognition of the organism, the more efficient is its defense system. There are two types of antigen recognition molecules in the immune system; the immunoglobulin produced by B cells and the T cell antigen receptor. These molecules have variable (V) regions which recognize antigens and constant (C) regions which mediate physiological functions. The latter provide additional diversity to the immune system. For example, the same heavy chain (H) V regions recognizing influenza hemagglutinin are expressed as the mu, gamma, epsilon, and a chains, each of which constitutes a different class of the immunoglobulin. Since the V and C regions are encoded by separate sets of DNA segments, it is the genetic variability of the V gene that determines the antigen recognition diversity. Recent molecular genetic studies on the immunoglobulin gene and the T cell antigen receptor gene indicate that the V region genes of both molecules produce somatic as well as evolutionary variations. Somatic variations include joining of two or three germ-line segments by site-specific recombinations and somatic base replacements. Evolutionary variations include gene duplication, segment transfer (gene conversion), mutational drifts, and so on. Since genetic events, be it somatic or evolutionary, tend to be random, not only useful but also harmful immunoglobulins are produced. Therefore, appropriate selection, be it positive or negative, is a prerequisite for the proper function of the genetic diversity of the immune system as the defense mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)