The major histocompatibility complex, or human leucocyte antigen (HLA) region, consists of genetically determined antigens, situated on chromosome 6. Found in most tissues, though to a diﬀering extent, the four gene loci are known as A, B, C, D, while the individual alleles at each locus are numbered 1, 2, 3, etc. The number of possible combinations is thus enormous, and the chance of two unrelated people being identical for HLA is very low.
HLA incompatibility causes the immune response, or rejection reaction, that occurs with unmatched tissue grafts. Strong associations between HLA and susceptibility to certain diseases – notably the AUTOIMMUNE DISORDERS such as rheumatoid arthritis, insulin-dependent diabetes, and thyrotoxicosis – have been described. Certain HLA antigens occur together more frequently than would be expected by chance (linkage disequilibrium), and may have a protective eﬀect, conferring resistance to a disease. (See IMMUNITY.)