The Bombay blood group is a rare exception to the commonly accepted ABO blood types. It is observed to occur in 1 out of every 250,000 people except in parts of India where the incidence has been observed to be as much as 1 in 7600. The rare designation was first identified in Mumbai, also known as Bombay - thus the name of the blood group. The blood type is thought to occur in only those of (Eastern) Indian descent.
Blood types are actually ways of differentiating the type of antigens on a person's red blood cells. Being able to match these during donation and transfusions is important because of potential rejection by the immune system. The Bombay blood group is missing an antigen present on cells of the ABO group, the H antigen. The H antigen is a carbohydrate known as fucose.
The ABO blood types, discovered around 1900, consist of A and B antigen which are derived from H antigen, also known as the H substance. Type A has A antigen, type B has B antigen, and type O has neither A nor B antigens. The difference though is that in type O, the H antigen is present, it just is not changed by enzymes and used in blood cell recognition. In Bombay blood type (phenotype hh or Oh) there is no H antigen and those individuals actually have antibodies against the H substance. Bombay blood cells will act like type O when tested for antigens, but the tests currently only look for A and B groupings. Proper blood typing will not identify the Bombay blood group. Specialists point out that reverse grouping, whish are cross comparisons with type O, or serum grouping would have to be done to determine if a person had the rare blood type. Those with the potential can be tested and then stock their own blood in blood banks for future use in surgery or transfusions. There was also speculation in 2004 about developing cryopreservation facilities for rare blood types to protect those who have them, especially in India where it is more likely to be a problem.
Bombay blood groups can donate to any other blood group (with Rhesus compatibility positive or negative) because there is no fear of an immune reaction against antigens, but they must receive only hh blood otherwise face an hemolytic transfusion reaction which is often fatal. The hh phenotype is the inheritance of two recessive alleles (h) of the H gene and is attributed to a deficiency of the enzyme known as fucosyl transferase. The mutation was discovered only a few decades ago and is thought to be the result of noble inbreeding or the former segregative caste system of India.