Human lymphocyte histocompatibility antigens were first detected as surface antigens (glycoproteins) on peripheral blood lymphocytes. Later, the HLA regions which codes for the antigens were localized to chromosome 6. Because the HLA complex is inherited intact as two haplotypes (one from each parent), siblings have 1 in 4 chance of being HLA-identical. HLA antigens are of clincially significance because these antigens elicite the strongest immunologic response, therefore, it is responsible for rejection of transplanted tissue and graft versu host reactions. It is also the cause of hemolytic diseases of newborn, erythroblastosis fetalis, kernicterus, and tranfustion reactions with poorly matched or mismatched blood. Furthermore, a host of clincial disorders are associated with HLA is various degrees, e.g. ankylosing spondylitis is associated with HLA B27.