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.