What is Mumbai blood group?

People with a rare Mumbai phenotype have no A or B antigens because there is no H protein on red blood cells. But their serum will produce antibodies against substance H, which exist in all blood types except Mumbai phenotype. Therefore, people with Mumbai phenotype can only accept blood of the same blood type.

Because Mumbai blood type is very rare, and they can't accept people with other blood types, people with Mumbai blood type who are in urgent need of blood transfusion are likely to find no blood donors.