Bombay phenotype

A type of blood group/blood type classification. People with Bombay phenotype are blood type O_h and are genotypically homozygous for the silent h allele (hh) of the H substance.

The rare individuals with Bombay phenotype do not express H substance on their red blood cells and therefore do not bind A or B antigens. Instead, they produce antibodies to H substance (which is present on all red cells except those of hh phenotype) as well as to both A and B antigens and are therefore compatible only with other hh donors.

Practically speaking, individuals with Bombay phenotype blood groups are only compatible blood donors for other Bombay phenotype individuals. Given that this condition is very rare to begin with, if you have this blood group and you need an urgent blood transfusion, you might be simply out of luck, as it would be pretty unlikely that any blood bank would have any in stock.

Bombay blood (hh) is the rarest of the rare blood types, essentially being completely outside the ABO system that the rest of humanity has. They do not have the A or B antigens, and instead of having the H antigen which defines type O, they actually produce antibodies to H.

This blood type was first discovered in Bombay in 1950 and that is where the name comes from. People who have this type of blood can only receive blood from others with the same type of blood, and there are very few of them. One article about a boy in India with this blood type states that there are only 57 people in India with Bombay blood.