There are three types of transplantation: autotransplantation (transplantation of self-tissue), allotransplantation (transplantation of non-self tissue from another non-identical individual of the same species), and xenotransplantation (transplantation of tissue from another species). With the exception of autotransplantation, all types of tissue transplants have a risk of undesired immune responses that result in the rejection of the transplanted tissue or disease. Both of these complications that can arise are a result of genetic incompatibility that is recognized by immune cells. Success of transplantation is dependent upon the similarity (histocompatibility) of the antigens of the donated tissue with that of self-antigens. Similarity at certain genetic sites (loci) are more important (major histocompatibility complex - MHC) than others (minor histocompatibility antigen MiHA) due to their broad expression on various types of tissue and their frequent interaction with immune cells.
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