Abstract
The major histocompatibility complex (MHC) was discovered originally as a genetic locus controlling rapid rejection of tissue grafts. Subsequently, study of antibody responses in vivo and T-cell responses in vitro to MHC antigens identified the presence of a number of closely linked loci within the MHC. Immune response (Ir) genes also mapped to the MHC. The discovery of MHC restriction and the molecular identification of MHC genes and their products has led to a unified theory of the principal function of MHC molecules to act as guidance molecules for T-cell responses. Additional functions are suggested by their association with cell surface receptors.