The Major Histocompatibility Complex: Influence on Immune Reactivity and T‐Lymphocyte Activation

The genetic region that influences graft rejection in animals and man is known as the major histocompatibility complex (MHC). The gene products of this complex (15) exert a profound influence on the activation of T lymphocytes. At least three effects of the MHC on T cells deserve special mention. First, the frequency of T lymphocytes that respond to cell surface alloantigens coded by the MHC is 100 to 1000 times greater than the frequency of T cells that respond to other antigens (27, 35). Second, the extent of a T‐cell response to certain antigens is controlled by genes that have been mapped in the I region of the MHC (3). Third, it has recently been shown that the MHC imposes constraints on the activities of sensitized T cells (7). The aim of this article is to review briefly some of the recent work performed in mice which allows the formulation of a model that may be used to explain tentatively the relationship between MHC gene products, T‐lymphocyte activation, and immune reactivity.