Histocompatibility
Isoimmunity is the reaction of the immune system to a different individual of the same species. This is what occurs in the rejection of a tissue or organ transplant.
The major histocompatibility complex, or MHC is a group of genes that code for "self" proteins. There are millions of different combinations of MHC genes.
- The antigens involved in transplant rejection are the HLAs (human leukocyte antigens), which are part of the MHC.
Transplant success will depend on the similarity of tissues, as killer T-cells will attack any foreign tissues as soon as they detect it. As such, there are four "classes" of transplant, from most likely to succeed to least likely to succeed :
- Autograft
: tissue from one site on a person transplanted to another site on the same person. MHC proteins are the same.
- Isograft
: tissue donated between genetically identical persons (identical twins). MHC proteins are the same.
- Allograft
: tissue donated between two individuals of the same species. ABO, HLAs and other blood group antigens must be matched, and at least 75% MHC matching before there is a "good chance" of the transplant working.
- Immunosupresssant drugs such as cyclosporine or corticosteroid drugs (prednisone) are typically given to prevent rejection
- Xenograft
: transplants between species. Virtually no MHC compatibility.