Human Leucocyte Antigens are varieties of genetically determined ANTIGEN situated on chromosome 6 and present on the surface of most body cells. They comprise the major reason for incompatibility between cells of one person and another. The number of possible combinations of these antigens is enormous, so 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.)