Must be exclusively of Pakistani, Indian or Iranian descent, meaning the donor's birth parents are both 100% Pakistani, Indian or Iranian.
To be compatible with Zainab, an ideal blood type will be missing the Indian B antigen or else chances are that she would reject the blood.
Zainab's tumor was found in her stomach two months ago, but doctors believe it may have been growing undetected for nearly ten months. This makes finding a match for her life-saving transfusions hard. "The possibility of us finding a compatible donor for this little girl within the right ethnic group is less than 4 percent".
But of those, only three donors have her rare blood type - including one in London. 'This was the worst thing we were expecting'.
"We were all crying.", said Zainab's father Raheel Mughal in a video.
According to the Mayo Clinic, neuroblastoma is a type of cancer that develops out of immature nerve cells that are formed in many areas of the body.
Neuroblastoma most often occurs in infants and young children, and accounts for about 6 percent of all cancers in children.
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The cancer can spread to tissues beyond the original site, including bone marrow, bone, lymph nodes, liver and skin.
OneBlood is working closely with other blood centers and the American Rare Donor Program (ARDP), an organization that searches the world for rare blood donors. The 2-year-old neuroblastoma patient has a rare blood type because of a genetic mutation.
The only people who are likely to be a match for Zainab are people of Pakistani, Indian or Iranian descent, OneBlood said.
The problem is that Zainab has a rare blood type. As of OneBlood's December 3 release, more than 1,000 donors have been tested and three have been identified.
The missing antigen "is so rare that honestly this is the first time I've seen it in the 20 years I've been doing this", said OneBlood lab manager Frieda Bright.
OneBlood is coordinating compatibility testing and asks that prospective donors specify that they are looking to donate for Zainab, so the blood can be tagged for testing.
"My daughter's life very much depends on the blood", Mughal said, describing the plea for help as a "humble request" from his heart.