Referring to the over 100 antigens on the surface of human body tissue cells that can be recognized by some kinds of leukocytes and potentially lead to the rejection of these tissues if they are alien. The HLA system has the most genes of any other known human multiple-allele series. There are at least 30,000,000 possible HLA genotypes. When organ transplants fail, it is usually due to HLA incompatibility. This can be prevented by making sure that the donor and the recipient have the same or very similar HLA types. Immunosuppressant drugs also are now available to restrain the HLA system and thereby allow successful transplants when there is an HLA incompatibility between donor and recipient. HLA stands for "human leukocyte antigen".