Bombay Blood group, is the rarest blood group. This blood phenotype was first discovered in Bombay, now known as Mumbai, in India, by Dr. Y.M. Bhende in 1952.
The classic Bombay phenotype has only been reported in those of Indian descent. It is quite rare with an incidence of 1 in 250,000. The bombay phenotype is characterized by mutations in fucosyltransferase. Fucosyltransferase is an enzyme that adds sugars to molecules exposed on the red blood cell surface. These sugars are used to determine the ABO blood type. Therefore a person with Bombay has the phenotype of O blood even though genetically they might have a different blood type.
The first person that was discovered to have the Bombay phenotype seemed to have an interesting blood type that reacted to other blood types in a way never seen before. The serum contained antibodies that reacted with all red blood cells’ (RBCs’) normal ABO phenotypes. The red blood cells appeared to lack all of the ABO blood group antigens plus an additional antigen that was previously unknown.
Individuals with the rare Bombay phenotype (hh) do not express H antigen (also called substance H), the antigen which is present in blood group O. As a result, they cannot make A antigen (also called substance A) or B antigen (substance B) on their red blood cells, whatever alleles they may have of the A and B blood-group genes, because A antigen and B antigen are made from H antigen. For this reason people who have Bombay phenotype can donate RBCs to any member of the ABO blood group system (unless some other blood factor gene, such as Rhesus, is incompatible), but they cannot receive blood from any member of the ABO blood group system (which always contains one or more of A and B and H antigens), but only from other people who have Bombay phenotype.
Receiving blood which contains an antigen which has never been in the patient’s own blood causes an immune reaction due to the immune system of a hypothetical receiver producing immunoglobulins not only against antigen A and B, but also against H antigen. The most common immunoglobulins synthesized are IgM and IgG (and this seems to have a very important role in the low frequency of hemolytic disease of the newborn among non-Bombay offspring of Bombay mothers).
It is very important, in order to avoid any complications during a blood transfusion, to detect Bombay phenotype individuals because the usual tests for ABO blood group system would show them as group O. Since Anti-H immunoglobulins can activate the complement cascade, it will lead to the lysis of RBCs while they are still in the circulation, provoking an acute hemolytic transfusion reaction. This, of course, cannot be prevented unless the lab technologist that is involved has the means and the thought to test for Bombay group.