Evolution and developmental regulation of the major histocompatibility complex.

The ontogeny and evolution of the major histocompatibility complex (MHC) are blossoming fields that grant insight into the origins of the adaptive immune system and into the strategies adopted by particular groups of vertebrates for expression of MHC during development. This review surveys general topics concerning MHC evolution, with special emphasis on the significance of linkage of gene families within the MHC; a model is proposed in which the MHC class III region is the "primordial immune complex" with its members giving rise to classical MHC molecules. The developmental expression of MHC, both of the classical and non-classical genes, is described in detail with a concentration on differential expression by extraembryonic tissues in mammals and by tissues in "transition" during metamorphosis in amphibians.