A patient's best opportunity to identify a perfectly matched marrow donor is found among siblings. There are six primary Human Leukocyte Antigens (HLA antigens) which must match a patient's in order for someone to be considered a potential marrow donor for that patient. These antigens are named by alpha characters and reported as numbers. Everyone has two "A", two "B", and two "DR" antigens. An example of an HLA type would be A2 A3 B7 B8 DR2 DR7. These antigens are inherited in sets of three...one "A", one "B", and one "DR" antigens from each parent.
These sets remain intact and are passed from generation to generation. With each parent having two sets of A, B, and DR antigens their children have a 1 in 4 chance of matching each other. [If four children in a family did not match each other,the fifth child would have to match one of the first four] In other words sisters and brothers have a 25%chance of matching each other. Immediate family members (mother, father and sisters and brothers) are the first to be tested when a patient is in need of marrow transplant.
If none of these prove to be compatible then a family study of extended relatives can be done. This presents a slightly better chance for finding a match than the unrelated donor search. Because the sets of antigens remain intact the same sets of antigens passed to the patient by his/her parents can also be found in aunts and uncles of the patient. If, by chance, aunts or uncles marry someone who carries some of the same antigens in the other half of the patients type, those aunts or uncles could have a child (a first cousin of the patient's) who could match. This could also happen with the aunts and uncles themselves.
Although the odds of finding an extended family match for a patient are not great, it is a stone which probably should not be left unturned. The American Registry has identified extended family matches for some of the patients for whom these studies have been done...1st cousins, uncles, and in one case, a great-aunt. In order to conduct a family study in an efficient and economically feasible manner, blood relatives are usually tested first. Those relatives who carry one of the sets of antigens are contacted and arrangements are made to test either their spouse or children. If there are several children, or if the children are young, testing the spouse can help determine whether it would be advisable to test the children.
If a related donor is identified, the Registry will be happy to work with that person and the family to explain the harvest process in detail, to put the person in touch with a former donor, and to assist in any way necessary through the remaining compatibility testing and marrow harvest process.
