The degree to which tissue from one organism will be tolerated by the immune system of another organism. For any animal, it is essential that its immune system can distinguish its own tissues from foreign cells or tissues, so that only the latter are attacked. This self-recognition is achieved principally by a set of marker molecules, called histocompatibility proteins (or histocompatibility antigens), which occur on the surfaces of cells. These proteins (in humans also called human leucocyte antigens, or HLAs) are encoded in vertebrates by a cluster of genes called the major histocompatibility complex (MHC). Each species has a unique set of MHC proteins, and there is also wide variation within any given species. This explains why in human transplantation it is very difficult to match donor and recipient tissue exactly (see hla system). MHC proteins also play a vital role in the immune responses of lymphocytes, notably by enabling T cells to identify foreign antigens. The MHC encodes two distinct classes of histocompatibility proteins, both of which are glycoproteins (see mhc class i protein; mhc class ii protein).