The TCR is noncovalently associated with five additional integral membrane proteins. The relative ability of each individual to respond to the antigenic universe is inherited to a large extent. Lymphocytes neither reproduce in uncontrolled fashion nor continuously produce increasing quantities of antibodies or lymphokines. A decline in functional expression follows primary and secondary immune responses. Hence specific immune responses must be down-regulated in some fashion.
Several mechanisms operate to modulate immunological expression.
Two broad categories of genes contribute to the control of immune responses. The first category of genes, receptor genes, controlling B and T cell receptors to a particular foreign antigen. This category is composed of two types of genes. Ir genes either can be part of the MHC or be unrelated to it. Ir genes of the MHC, which encode class II molecules, primarily influence responses involving T helper cells and sensitized T delayed hypersensitivity cells, While those that encode class I molecules, principally control T suppresser and T cyotoxic cells.