The major histocompatibility complex (MHC) is a group of closely linked loci present in remarkably similar form inall mammals and perhaps in all vertebrates. It plays a still imperfectly understood but clearly important role in immune phe-nomena. Becauseof the unusual concen-tration of similar genes, I referred to it in 1968 as a supergene (1). Bodmer (2) has gone me one better, calling it a super supergene. The term is not inappropri-ate because we now know that the MHC contains at least four gene clusters, each with its own type of end product and its own specific effects on the immune re-sponse.

The MHC was originally discovered because of its role in the rejection of transplants made between incompatible individuals. Genes competent to play this role in the appropriate experimental or surgical context are called histocom-patibility or H genes. An influence on transplants probably is entirely irrele-vant to the true function of such genes, but the influence does give the geneticist a handle by which to study them. It was by this route that, over a period ofa good many years, I became involved first in immunogenetics and then in the new and fascinating area of cellular immunity.