Genetics of transplantation: the major histocompatibility complex.

A genetic region called the major histocompatibility complex of MHC (which plays an important role in the control of graft survival) has been defined in a large number of different species. Several different loci of the MHC have been described, including loci coding for transplantation antigens. These antigens can be divided into two categories: first, the SD antigens that were originally defined serologically and that appear to function as targets for the killer lymphocytes involved in the rejection of a graft and second, the LD antigens that were originally defined by lymphocyte response in the mixed leukocyte culture test and that stimulate helper T lymphocytes. In addition, there are genes for other antigens (referred to as immune response-associated or Ia antigens) and genes that control the ability of an animal to respond immunologically to antigenic stimuli, the immune response or Ir genes. There is evidence for epistatic interaction between these genes in that immune recognition of LD and SD antigens lead to a more pronounced development of cytotoxic (killer) cells than does stimulation by either antigen alone. In addition, the genetic control of immune responsiveness appears, at least in some systems, to reside in two very closely linked genes that may function better in the cis than in the transposition. This latter finding suggests a possible explanation for the strong linkage disequilibrium found between genes of the MHC. A number of new tests have been described to define the antigens of the MHC. Results of these tests have been used for studies of transplantation immunology but in addition to study the very strong associations that exist between some of the MHC antigens in man and various diseases.