the human major histocompatibility complex , located on the short arm of chromosome 6. Five loci have been identified, designated HLA-A, HLA-B, HLA-C, HLA-D, and HLA-DR (D related), each with multiple alleles , designated by numerals (such as HLA-B5). Provisionally identified alleles are designated by the letter w (such as HLA-Bw22). Called also human leukocyte antigens . The HLA-A, -B, and -C loci gene products are cell surface antigens found on the surface of all nucleated cells. They are the most important antigens in transplant rejection , when donor and recipient do not match. The HLA-A, -B, and -C antigens are recognized by killer T cells that lyse the target cells and are also involved in the cell-mediated lysis of virus-infected cells. The HLA-D and -DR loci gene products are cell surface antigens of B lymphocytes and macrophages and are involved in antigen presentation and other forms of cooperation between immunocompetent cells as well as in stimulation of transplant rejection .