The major histocompatibility complex (MHC) has attracted much attention because of its
immense polymorphism, its importance in transplantation, and its indisputable role in
disease susceptibility in humans and animals. Two classes of MHC have been identified.
Class I MHC consists of two non-covalently attached polypeptide chains, the alpha chain
and beta(2)-microglobulin. The alpha chain is approximately 40-50 kDa [kiloDaltons] in
size, membrane bound, and is encoded by several genes within the MHC. Class II MHC also
consists of two membrane-bound polypeptides, alpha and beta, both of which are approximately
30 kDa in size and are encoded by genes within MHC. In fish, genes of polypeptides which
contain MHC class I and class II have been identified and investigated. The size and
organization of fish MHCs genes are similar to those of mammals. There are several loci
encoding fish MHC and there is a high degree of polymorphism observed in the putative
peptide-binding region of class I alpha, class II alpha and beta chains. Thus, in spite of
great sequence divergence between fish and mammalian MHC genes, their overall organizations
are well conserved.