Definition: Histocompatibility antigens is a blood test that determines a person's major histocompatibility antigens.

Why the test is performed: HLA (human leukocyte antigens) are proteins found in the membranes (outer coating) of nearly every cell in the body (all cells that have a nucleus). These antigens are in especially high concentrations on the surface of white blood cells (leukocytes).

HLA antigens are the major determinants used by the body's immune system for recognition and differentiation of self from non-self (foreign substances). There are many different major histocompatibility (HLA) proteins, and each person possesses only a small, relatively unique set that is inherited from their parents. It is unlikely that 2 unrelated people will have the same HLA make-up.

Children, on average, will have one-half of their HLA antigens that match one-half of the mother's antigens. The other one-half of the child's antigens will match one-half of the father's antigens. This is particularly important in identifying good matches for tissue grafts and organ transplants, such as a kidney transplant or bone marrow transplant.

Many HLA molecules exist, but some are of special interest because they are more common in certain autoimmune diseases. For example, HLA-B27 antigen is found in 80-90% of people with ankylosing spondylitis and Reiter's syndrome, and can aid in the diagnosis of these diseases. HLA-B27 is also present in 5-7% of people without autoimmune disease. Thus, the mere presence of this HLA molecule is not indicative of disease.

HLA types can also used to determine relationships between children and parents when such relationships are in question. However, newer, more specific genetic testing is now available for this purpose.

Source: MedlinePlus Medical Encyclopedia