A histocompatibility antigen blood test looks at proteins called human leukocyte antigens (HLAs), which are found on the surface of nearly every cell in the human body. HLAs are found in large amounts on the surface of white blood cells. They help the immune system tell the difference between body tissue and foreign substances.

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

No preparation is necessary.

When the needle is inserted to draw blood, you may feel moderate pain, or only a prick or stinging sensation. Afterward, there may be some throbbing.

The results from this test can be used to identify good matches for tissue grafts and organ transplants, such as a kidney transplant or bone marrow transplant.

Each person has a small, relatively unique set of HLAs that they inherit from their parents. Children, on average, will have half of their HLAs match half of their mother's and half of their HLAs match half of their father's.

It is unlikely that two unrelated people will have the same HLA makeup.

There are three main groups of HLA:

• HLA-A
• HLA-B
• HLA-DR

However, each group contains many different HLA proteins.

Some HLA types are more common in certain autoimmune diseases. For example, HLA-B27 antigen is found in many people (but not all) with ankylosing spondylitis and Reiter syndrome.

This test may also be 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.

Each person has unique HLA antigens, although identical twins may match each other.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)