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.
The 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.
Ethnicity plays an important role in the process of finding donors. Since HLA markers are inherited from both parents, there are strong ethnic ties to a person’s HLA pattern. In other words, while anyone, anywhere can potentially be a match, the best chance for a good HLA match comes from a donor who shares a patient’s ethnic background. Some ethnic groups have greater diversity of HLA types than others, however, and less-common tissue types are more challenging to find a match for. A 2008 NMDP study indicates that even when ethnic minorities do find donors, they are 2-3 times more likely to have a less-matched donor—and thus, worse prognosis—than Caucasian patients, due to the wider diversity of HLA types. Currently, South Asians represent just 2% of registered donors, both in the U.S. and Canada (figure, below).
Why are minorities underrepresented in donor registries? Primarily, most people do not know that such registries even exist until someone close to them needs a transplant. Understanding this, the NMDP and OneMatch have recently developed broad programs to help improve public knowledge. Secondly, there are many misunderstandings about the “risks” of the registration and donation procedures. A 2005 study conducted by researchers at the University of Pittsburgh found that Asian Americans are more ambivalent about joining registries, and those who join are more anxious about the donation process than any other ethnic group.