Two-year-old Zainab Mughal has neuroblastoma and will need blood transfusions during her treatment. But the little girl has a genetic variation in her blood that makes finding a compatible donor extremely difficult. Ross Herron, the chief medical officer of the West Division of American Red Cross Blood Services, said there is a lot more to blood compatibility than just type.

"We have the common blood groups of group A, B, AB, and O, and we have Rh positive and Rh negative and everybody is used to hearing about those, but we have hundreds of other antigens that fall into various families of red cell antigens, and each one has their own genetic frequency that are found in various ethnic groups," Herron said.

Now, a worldwide search is underway to help this Florida girl's fight against cancer.

Zainab's blood is missing an antigen — Indian B — and she will only find a match in her ethnic community, so matches will only be found in people of 100-percent Pakistani, Indian, or Iranian descent. Even then, only 4 percent of that population is likely to match.

An organization called One Blood and the Red Cross'sRare Donor Program are working together to help Zainab to get the blood she needs. Three donors have been found, but she'll need several more to get through treatment.

Heron said donating blood is critical for people of all ethnic groups, but said there can be shortages even if the blood contains no genetic rarities.

"Among Caucasian Hispanics, the rate of O positive is about 55 to 60 percent, and among our donors that are Caucasian non-Hispanic, the rate is about 45-percent, so we have a disproportionate need for O positive donors,” Herron said.

Herron added that B negative blood is often in short supply because it is in disproportionate demand in the African-American community. So, Herron said, everyone of every ethnic background should donate blood.

"We need a diverse donor population to be donating for the diverse patient population that we have."

Bonnie Petrie can be reached at bonnie@tpr.org or on Twitter @kbonniepetrie