What is a match and how does it work?

If you are among the generous minority of people who donate blood, you will know that for a transfusion to work, the blood type of the donor must be compatible with that of the recipient. Finding a compatible unit of blood for a patient is a relatively easy task (though there is always a need for more blood donors), since in the familiar ABO-Rh typing system, there are only eight major blood groups.

Matching stem cell donors to recipients is much more complex than blood typing, and uses a set of markers found on most cells in the body, called “human leukocyte antigens” (HLA). These cell-surface proteins distinguish cells belonging to our own bodies from those that don’t, so that our immune systems can attack foreign invaders while leaving our own cells alone. We each inherit six major pairs of HLA markers from our parents, half of each pair from our fathers, the other half from our mothers.

Each marker, however, can have hundreds of different forms in the general population. If each of these 12 markers were inherited independently, there would be over 900 trillion possible combinations, and nobody would ever find a match. Fortunately, HLA is inherited in sets, which considerably improves the odds of matching. Still, the odds could be much better: the current likelihood of an American or Canadian patient finding a match ranges from 32% to 88%, depending on the patient’s race—with ethnic minorities at the low end of this range.

In the GenesThe best source of finding a match is from within a patient’s own family. Due to the way HLA is inherited, each sibling of a patient has a 25% chance of being a perfect donor match (figure, left). Overall, nearly 30% of patients are lucky enough to find a related donor. Unfortunately, this means that more than 70% of patients must search for an unrelated donor to save their lives. This is where stem cell registries play a critical role.

Registries are organizations that help recruit potential stem cell donors, perform the necessary tests to identify a donor’s HLA type, and store this information in databases that allow patients to search for matches. The first stem cell registry (The Anthony Nolan Trust) was established in England in 1974 by Shirley Nolan, whose infant son was diagnosed with a rare immune system disorder. Although Anthony passed away at age seven, Ms. Nolan’s vision has spread globally, and many countries now operate at least one registry; these include the Be The Match RegistrySM operated by the National Marrow Donor Program (NMDP) in the U.S., and the OneMatch registry in Canada. An international organization, Bone Marrow Donors Worldwide (BMDW), collects data from national registries which can be viewed by all member registries globally. In March 2009, BMDW reported that it has data on 13 million prospective donors.

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