Immunology of transplantation

Abstract

The immune response to transplants is fundamentally no different from that to pathogens. The main difference is in the nature of the antigens. Cells of a transplant bear both class I and class II major histocompatibility complex (MHC) molecules on their surface. Because of the enormous variation of class I and class II genes between individuals, unless the transplant is from a relation, it almost certainly bears variants of class I and class II molecules that are not possessed by the transplant recipient. For reasons that are not fully understood, foreign MHC molecules stimulate a much higher number (10–100-fold) of T cells than foreign antigen presented by self-MHC and therefore the immune response against foreign MHC transplantation antigens is much stronger than that seen against pathogens. In addition to foreign MHC molecules (major transplantation antigens), transplants have many proteins that are a variant of the recipient's equivalent proteins. These are called minor transplantation antigens and, though they do not stimulate as strong an immune response as foreign MHC, they can stimulate rejection of the transplant, unless the response against them is suppressed.