Structure and function of the major histocompatibility complex in domestic animals.

The major histocompatibility complex (MHC) is a genetic region that has been intensively studied for the past 2 decades. Interest in the MHC has been high because of (i) the particular involvement of the MHC in transplantation reactions, including organ allograft rejection in human beings; and (ii) the more general role of MHC gene products in the genetic control of immune responses in all mammals. The MHC has several remarkable properties that include a distinctive genetic structure which has been well-preserved through evolution, and the extreme plasticity of form of the principal MHC genes, which can coexist within a single species in 30 or more allelic forms. The genes of the MHC regulate cell-cell interactions of various types within the lymphoreticular system, and thus function as the so-called "immune response" genes that have been described in mice, rats, and guinea pigs. In human beings, the "disease associations" demonstrated between MHC alleles and various pathologic conditions are probably manifestations of abnormal functions of immune regulation governed by the MHC. Studies of the MHC in domestic species are still in their infancy. However, investigations of the MHC have been carried out in swine, cattle, horses, sheep, goats, dogs, and chickens. Further research on the MHC of domestic animals is merited, both for its contribution to the overall understanding of the biological significance of the MHC and for its practical application in clinical veterinary medicine.