Immunogenetics and Immunologic Mechanisms of Rejection

The major histocompatibility complex (MHC) is a group of genes found in a region on the short arm of the sixth chromosome in man (Fig. 2.1).1 This region codes for histocompatibility antigens that are known to be the strongest barriers to transplantation of organs from one human individual to another2; these antigens elicit strong immune responses when they are not matched. Experience in kidney allograft transplantation has demonstrated that matching for these antigens within families and between unrelated individuals (as in cadaveric kidney transplantation) significantly reduces the strength of the immune response and leads to superior graft survival.3 This is also the case in bone marrow transplantation, wherein successful engraftment relies on both the absence of rejection and the absence of a graft-versus-host reaction. To achieve this, the histocompatibility antigen discrepancy must be minimal when donor cells scrutinize the recipient and when recipient cells scrutinize donor cells.4 All vertebrate animals that have been studied, including rodents, dogs, lower primates, and swine, have a major histocompatibility complex. In man, the glycoprotein gene products of the major histocompatibility complex are known as human leukocyte antigens, or HLA.