The essential procedure for matching the tissue of a recipient in need of transplanted tissue or organ to that of a potential donor. Unless there is a reasonable match, the recipient's immune system (see IMMUNITY) will reject the donor's organ. The main factors that are relevant to an individual's reaction to donor tissue are called histocompatibility antigens (see ANTIGEN). These are mostly human leucocyte antigens (HLAs – see HLA SYSTEM) present on the surface of cells. HLAs are inherited and, like fingerprints, unique to an individual, although identical twins have identical HLAs and hence are perfect matches for TRANSPLANTATION procedures.