A complex of genes on chromosome 6 that code for the antigens that determine tissue and blood compatibility. In humans, histocompatibility antigens are called human leukocyte antigens (HLA) because they were originally discovered in large numbers on lymphocytes. There are thousands of combinations of HLA antigens. Class I MHC antigens (HLA-A, HLA-B, and HLA-C) are found on all nucleated cells and platelets. Class II antigens (HLA-DR, HLA-DQ, and HLA-DP) are found on lymphocytes and antigen processing cells and are important in the specific immune response. In tissue and organ transplantation, the extent to which the HLA or “tissue type” of the donor and recipient match is a major determinant of the success of the transplant.
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