hh, or the Bombay blood group, is a rare blood type. This blood phenotype was first discovered in Bombay by Y. M. Bhende in 1952. It is mostly found in the Indian subcontinent (India, Bangladesh, Pakistan and Iran).
people who have Bombay phenotype can donate red blood cells to any member of the ABO blood group system (unless some other blood factor gene, such as Rh, is incompatible), but they cannot receive blood from any member of the ABO blood group system (which always contains one or more of A, B or H antigens), but only from other people who have Bombay phenotype.
the usual tests for ABO blood group system would show them as group O.
Maybe not surprising in retrospect but blood typing is far more complex than I realized before going down the rabbit hole. The general article on blood types is also quite interesting. When you go beyond ABO and Rh typing there’s far more variety than what we typically think of for blood types.
As of June 2025, 48 blood-group systems have been identified by the International Society for Blood Transfusion in addition to the ABO and Rh systems.
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Cool, thanks for the info.
Sounds sort of like double secret type O blood
Wait… So someone with group AB could receive hh blood?
Yes, assuming the Rh factor aligns, but the inverse is not true.
