Human Leukocyte Antigens (HLA) are known as the "self" molecules, because they define the body's parameters as to what belongs there (your lungs) and what does not (rhinoviruses). They are ten genetically inherited proteins present on the surface of human cells. When your body's immune system (T-cells or B-cells) sees these proteins, it recognizes the cell as part of your body and leaves it alone. When they see proteins that are not compatible, they attack.

Because there are ten of these antigens and each type comes in a huge variety, your personal HLA code is very unique. This is what makes it difficult to match types for organ and bone marrow donation. With most organ donations, the worst thing that can happen is your body's immune system will reject the organ and attack it. While this is a very serious problem, a mismatched bone marrow donation is even more of a life threatening condition.

A bone marrow transplant is required when chemotherapy kills all the bone marrow in a patient's body. Since bone marrow produces the cells involved in the human immune system, this leaves the patient without an immune system, requiring a transplant to build a new immune system. The special problem here is that if the HLA type is mismatched, the body's new immune system will see the entire host body as a foreign substance and begin attacking everything it sees. This is called graft-versus-host-disease, or GVHD.

To prevent this, DNA testing on white blood cells is required to match at least six of the ten HLAs in the host and the donor. The specific antigens that are tested for are known as HLA-A, HLA-B, and HLA-Dr. A person has two of each type, inheriting one from the mother and one from the father. A perfect 10/10 match is ideal, but very unlikely. The chances of two unrelated people providing a match just for these six is 1 in 20,000. Even siblings have only a 25% chance of being a 6/6 match. For this reason, sometimes a 5/6 match is close enough if a 6/6 match cannot be found, and in rare circumstances a parent or sibling is considered a match with only a 3/6 (this is called a haploidentical transplant). Identical twins are, of course, a perfect 10/10 match.

Complicating matters is the fact that some of these antigens are specific to certain ethnic groups. For example, a person's tissue type might look like the following:
A2402, A3303; B4001, B5801; DR1501, DR1501
The homozygous DR1051, DR1501 is common among both Chinese and Greeks, and the A3303, B5801 antibodies are unique to the Chinese. This creates a special problem in multi-ethnic individuals, who often have trouble finding a compatible donor. Such is the case with Enrico Console, a Chinese-Italian child who was diagnosed with Acute Lymphatic Leukemia (ALL) in 1996. As of 2002, the Asian American Donor Program has not yet found a match for his HLA type despite a huge amount of publicity and free compatibility testing for multi-ethnic and minority donors. Testing generally costs about $85 in the United States.

The majority of the information in this writeup was obtained from the following websites: