THE major histocompatibility complex (MHC) is a cluster of genes controlling a variety of immunological and nonimmunological phenomena 1. Membrane-bound glycoprotein products of the MHC genes are of two classes: molecules of one class consist of 44,000 MW polypeptide chain which is non-covalently associated with a smaller (12,000) chain of β2- microglobulin1; molecules of the second class consist of two polypeptide chains of 33,000 and 28,000 MW1. The MHC was originally discovered in the mouse2, where it is referred to as the histocompatibility-2 or H-2 system3. Later, similar systems were described in man, rhesus monkey, chimpanzee, rat, dog, rabbit and guinea pig; and, in fact, may exist in most, if not all, mammalian species4. Furthermore, clusters of genes controlling graft rejection, mixed lymphocyte reaction (MLR), immune responsiveness, and complement activity have also been reported in the chicken5 and Xenopus6. It is tempting to speculate that these gene clusters controlling immune functions form a single evolutionary line from more primitive ancestral genes in lower vertebrates to the HLA complex in man and the H-2 complex in the mouse7. Proof of such a relationship exists for the MHC of man and mouse, where the H-2 and HLA systems show considerable amino acid sequence homology in both classes of molecules7. Because of the relative closeness of the two species (H-2 and HLA), homology is not surprising. We have investigated whether the homology extends to other vertebrate classes, such as birds: for example, is the chicken B system a true evolutionary homologue of H-2 or HLA?
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