Acronym for human leukocyte antigen. Histocompatibility antigens are proteins used by the immune system to differentiate self cells from non-self cells. The system of histocompatibility antigens found in humans is collectively called HLA, with one's HLA genes determining one's HLA type. HLA proteins can be found on the surface of all cells except for red blood cells, and if a transplant recipient receives an organ or tissue with a different HLA type, this will cause a rejection reaction whereby the body will attack the cells of the foreign organ or tissue, which it treats as an invader.

There are numerous antigens, and virtually millions of possible HLA types, due to the nature in which they are inherited from one's parents. HLA types are particularly important in regard to bone marrow transplants. Histocompatibility tissue tests are conducted to determine whether a donor and a recipient share a common HLA type; this is done by examining the lymphocytes in the subjects' blood.