If you become ill with a blood cancer or other disease that requires a stem cell transplant, here’s an uncomfortable fact: Your race matters. Diversity is a strength in much of life, but it’s a curse when finding a stem cell donor match.

For a successful transplant, donor and recipient must have nearly identical genes regulating certain immune cells. These genes evolved in response to the disease threats people faced long ago. “Tell me where your ancestors lived 500 years ago, and I’ll tell you who your potential donors are,” says Jeffrey Chell, an internist who leads the National Marrow Donor Program, also known as Be The Match.

African Americans have the most diverse genetics because their ancestors have been around the longest and because of intermixing with whites, Native Americans and Hispanics since Africans arrived in the Western Hemisphere. When all of humankind’s relevant genes are considered, there are 10 billion possible combinations. That means that “if your ancestors were on two or three continents,” Chell says, “it’s going to be harder to find a match.”

Genetics make the mathematics difficult for people of color. Cultural traditions, mistrust of medicine and ignorance about the need for donors make it worse.

“I didn’t know anything about bone marrow transplants until I learned that I might need one,” says Anthony Thomas, 49, a financial consultant from Ran­dalls­­town, Md., who has chronic lymphocytic leukemia. Among his African American friends and colleagues, there’s little awareness of the importance of donating, he says, although “you can bet that if Lil Wayne or Beyoncégot leukemia, that would change.”

Nina Louie, 33, eating a healthy meal to stay strong for her battle against lymphoma. (Matthew Louie)

“People tell me, ‘I’m praying for you,’ ” says Thomas, who spent July 18 on Capitol Hill lobbying against the sequestration of federal funds that have helped Be The Match enroll donors. “I tell them, ‘Prayer is great, but if you want to help me, start a donor drive at your church.’ ”

Thomas may not need a transplant for three to five years, he says. The situation is much more dire for Nina Louie, 33, whose parents are of Chinese descent and who has some Thai ancestors. She is suffering from advanced lymphoma, and chemotherapy is no longer an option.

Sorority sisters mobilize

Ironically, Louie and nine of her sisters in an Asian American sorority joined a stem cell registry 15 years ago as freshmen at Stanford University after learning that cancer patients of Asian descent lacked adequate donor options for transplants. Today, those nine women are battling to save their beloved sorority sister.

In two months, they have helped enlist nearly 8,000 potential donors through a social marketing campaign and ads on 350 billboards across the country. The message: “Save Nina.” The drive, which is aimed particularly at Asians and Asian Americans but asks everyone to register, has located potential donors for three other patients, all of Asian descent. But it has not yet turned up one for Louie. Hospitalized near her home in Los Angeles after 14 rounds of chemotherapy, she is spending as much time as she can with her husband, Matt, and their 2-year-old son, Donovan.

Be The Match, a nonprofit launched in 1986, provides potential access for people such as Thomas and Louie to the healthy cells of 22 million people around the world, about half of them in the United States.

For patients with leukemia, lymphoma and other immune disorders, bone marrow and peripheral blood stem cell (PBSC) transplants help destroy cancerous cells and replace immune cells killed off by chemotherapy. In PBSC, cells are filtered out of the donor’s blood and infused into the patient; in marrow donation, a needle is used to withdraw liquid marrow, which also contains stem cells, from inside the pelvic bone.

The first stem cell transplants were done in the 1950s. Last year, doctors performed more than 50,000 worldwide and the global total reached 1 million transplants. Because of potentially fatal risks, the procedure is performed only on people in dire condition. A recent study in the Journal of Clinical Oncology found that about 90 percent of patients survived at least 100 days after the transfusion, and about 63 percent for a year or more.

Family members are key

About a third of stem cell transplants are from young relatives of the patient, but in Louie’s case no familial match could be found. Doctors have told her she has a 1-in-20,000 chance of finding a match.

In addition to being disadvantaged by their genetic diversity, many minority-group members in the United States have been less likely than other Americans to donate or register for donation, as Louie and her sorority sisters learned years ago.

Be The Match has narrowed the donation gap: For example, African Americans have gone from being 5 percent of the registry to 9 percent in the past two decades. (They comprise 13 percent of the U.S. population.) Still, mostly for genetic reasons, about 93 percent of Caucasians will find a donor through the registry, Chell says, compared with 73 percent of Asian Americans and 67 percent of African Americans.

The disparity in successful transplants is even larger, because minorities who enter the registry are more likely to back out when a match is found and they are asked to donate, says Galen Switzer, a professor of psychiatry at the University of Pittsburgh. About 60 percent of whites go forward with the donation but only about 40 percent of racial minorities, he says, with slightly fewer African Americans going forward than other minority groups.

Nalini Ambady, a Stanford psychology professor born in India, received a diagnosis last fall of acute myelogenous leukemia. At Christmas, she and her family learned that 12 potential matches had been found. But six of the potential donors were ruled out for a variety of medical reasons. (This often occurs with people who joined the donor list years ago, before recent scientific discoveries that allow doctors to reject donors who would not be good matches.) Much to the family’s shock, the other six refused to donate. In July, another donor was discovered, and Ambady is expected to undergo a transplant soon.

Both PBSC and marrow transplant involve time commitments, some discomfort and short recovery periods that may discourage some donors. In some traditional Asian communities, beliefs hold sway that blood and organs are a gift from ancestors and that giving them away will bring bad luck or cause weakness.

Religion can be a barrier

Reluctance to donate stem cells because of religious beliefs, family pressures or mistrust of the medical system are the main reasons people back out, and this reluctance is more common among minorities than whites, according to a study that Switzer and colleagues published this year in the journal Blood.

Vivian Lam Coates met her friend Louie when they were pledging the Alpha Kappa Delta Phi sorority in 1998. “She was this tiny, bubbly, really positive girl; she was beautiful and really smart, an Econ major,” says Coates, a lawyer who lives in North Carolina.

The silly pledge things they did together remain in Coates’s mind — the marathon pledge-pin-wearing and sleepovers, the dumpster diving for a sister’s lost key chain, the time Louie passed out after a single glass of wine (alcohol intolerance being a common Asian genetic trait). And when an Asian American fraternity at Stanford organized a drive each year in honor of a pledge who had died of leukemia, they all signed up. “There was this idea that there weren’t enough Asian donors,” Coates recalls.

After college, Louie became a financial consultant in Thailand, and later she got an MBA at Harvard, where she met her husband. She was the first of the 10 sorority sisters to have a baby. In October, the sisters met for a 10-year reunion in San Francisco, but Louie didn’t make it. Her lymphoma had just been diagnosed. The disease went into remission after a few brutal months of chemo, but it recurred in February. In May, Louie was told that her only hope was a transplant.

Nothing is known about the three patients helped by the drive to find a donor for Louie because Be The Match keeps a firewall of anonymity between donors and recipients. Donors are updated on the progress of their recipients, however, and the two can learn each other’s identity after a year, if both consent.

As time speeds by without a match for Louie, her friends confront the possibility that they could end up helping others without being able to save their friend. “I guess it feels sort of bittersweet,” says Shen Li-Khong, another sorority sister. “She’s our first priority. We want a match for her. But if we can help people along the way, that also makes it better.”

Allen is a freelance writer and the author of “Vaccine: The Controversial Story of Medicine’s Greatest Lifesaver.” To learn more about Nina Louie’s situation and about registering to donate stem cells, go to www.savenina.com or www.bethematch.org.