Comparative genomics of the MHC: glimpses into the evolution of the adaptive immune system.

MHC gene organization (size, complexity, gene order) differs markedly among different species, and yet all nonmammalian vertebrates examined to date have a true "class I region" with tight linkage of genes encoding the class I presenting and processing molecules. Three paralogous regions of the human genome contain sets of linked genes homologous to various loci in the MHC class I, class II, and/or class III regions, providing insight into the organization of the "proto MHC" before the emergence of the adaptive immune system in the jawed vertebrates.