Bombay Blood Group
"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 RBCs normal ABO phenotypes. The RBCs 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 has 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 any member of the ABO blood group system's blood (which always contains one or more of A and B and H antigens), but only from other people who have Bombay phenotype."
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This blood group is very very rare. We have data of only 12 such donors in whole of Karachi.