Source:  BOMBAY BLOOD GROUP    Tag:  bombay phenotype
hh is a rare blood group also called Bombay Blood group. 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 (also called "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; receiving blood which contains an antigen which has never been in the patient's own blood causes an immune reaction. As a result, people who have Bombay phenotype can donate 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. The usual tests for ABO blood group system would show them as group O, unless the hospital worker involved has the means and the thought to test for Bombay group.
Individuals with Bombay phenotype blood group can only be transfused with blood from other Bombay phenotype individuals. Given that this condition is very rare, any person with this blood group who needs an urgent blood transfusion will probably be unable to get it, as no blood bank would have any in stock. Those anticipating the need for blood transfusion (e.g. in scheduled surgery) may bank blood for their own use (i.e., an autologous blood donation), but this option is not available in cases of accidental injury.
Patients who test as type O may have the Bombay phenotype if they have inherited two recessive alleles of the H gene, (their blood group is Oh and their genotype is hh), and so do not produce the H carbohydrate that is the precursor to the A and B antigens. It then no longer matters whether the A or B enzymes are present or not, as no A or B antigen can be produced since the precursor antigen H is not present.
Despite the designation O, Oh negative is not a sub-group of any other group. When Bombay blood group was first encountered, it was found not to contain antigens A or B and so was thought to be of group O. But experience showed that Bombay group patients could not even safely receive normal O-group blood, and this proved to be because they lacked the H antigen.
Because both parents must carry this recessive allele to transmit this blood type to their children, the condition mainly occurs in small closed-off communities where there is a good chance of both parents of a child either being of Bombay type, or being heterozygous for the h gene allele and so carrying the Bombay characteristic as recessive. Other examples may include noble , which are inbred due to custom rather than local genetic variety.