[The HLA system. An introduction].

The cellular identity is determined by the cell surface antigens. The recognition of self and non-self in vertebrates is mainly controlled by antigens and coded for by the major histocompatibility complex (MHC). Upon recognition of a foreign antigen, the immune system does not only initiate a reaction with the help of cytotoxic T-cells, phagocytes and humoral antibodies; memory cells are also generated, enabling a very swift and powerful response after repeated exposure to the same foreign antigens. These characteristics, although essential for the survival of the organism in a hostile environment, can markedly limit the life of useful and potentially lifesaving organ transplants.