It was a tall order: the Indian B antigen is known to be lacking in the blood of Iranians, Pakistanis and Indians, so donors had to have both parents from these populations. Two donors live in the United States, two in Britain and one in Australia. Most people, says Susan Forbes of One Blood, “don’t understand the need for a diverse blood supply.”

Publicity about Zainab’s case, though it was extreme in its rarity, helped raise awareness.

Yet it is a difficult message: that our blood is different, red and white cells both. When it comes to finding stem cells for bone marrow transplants, the search has to be equally discriminatory. This time the issue is HLA, the human leukocyte antigens present in white blood cells.

“The reason why ethnicity comes into the picture,” says Dr. Abeer Madbouly, a senior scientist at Be the Match, a program run by the National Marrow Donor Program, the largest stem cell donor registry in the world, “is that HLA encodes the immune system, and the immune system goes through particular conditions based on where you are.” Depending on the threat, each population will develop particular sets of HLA types. In a diverse population like that of the United States, finding a matched donor becomes more challenging for a patient with a mixed ethnic background.

“Let’s say you have someone with African roots and someone from Asian descent coming together, and then they have an offspring of mixed ethnicities,” Dr. Madbouly said. “You have an African HLA and an HLA type common in Asian areas coming together to form a new type of HLA that is not common in either.” Though Be the Match added nearly two million donors to its registry last year, only 30 percent were what Dr. Madbouly calls “diverse.” That’s not enough.

Zainab’s blood is rare, and so is her situation. What concerns blood bankers on a daily basis is a more common condition caused by uncommon blood. Sickle cell disease is predominantly found in African-Americans, and thalassemia among South Asians, and both conditions require precisely matched blood. But there is a shortfall between ethnic minority patients who need blood, and ethnic minority donors. In New York, Caucasians are 35 percent of the population but 58 percent of donors. Twenty-eight percent of New Yorkers are African-American but only 8 percent of the donors in New York are African-American, and that’s after five years of hard work and outreach by the New York Blood Center with its PreciseMatch campaign.

Even so, there was trouble when the Blood Center began in 2009 to offer the option to “self-declare” ethnicity on its donor forms. This was efficient: without a budget to precisely screen every donation, they could home in on antigens known to be specific to certain populations. At first there were problems, when staff members were initially upset by this apparent division of blood by ethnicity. “We didn’t educate the staff,” says Dr. Westhoff, “to know that we weren’t segregating the blood just to be segregating. We were doing it to send all the African-American units to the sickle program children because they were doing much better with blood that came from this same ethnic group.”