Hh antigen system
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.

# Transfusion compatibility
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.

# Genetics
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 (fucose) 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 it is required that both parents carry this recessive allele to transmit this blood type to their children, the condition mainly occurs in small closed-off communities  where the recessive gene has a chance to find two parents with this same blood type. Other examples may include noble families, which are inbred due to custom rather than local genetic variety.
The blood phenotype was first discovered in Bombay, now known as Mumbai, in India, by Dr. Y.M. Bhende.

# References in popular culture
Television: Yakitate Japan Episode 49 of Monoco Cup Arc  Reunion at the VIP Seat! Heaven Again!  the clown Pierrot has Bombay blood. He gets wounded in the course of the show, and being rare enough that according to the show only several hundred people in the world would have it. It turns out his father is the king of Monaco, who also has Bombay blood.
Television: Psi Factor The OSIR team encountered a young woman with the Bombay phenotype while working to solve the case of a haunting in the underground maintenance tunnels of a major city. The ghost would later be revealed as the spirit of her dead infant.
Television: Get Backers Episode 35  Get Back the Life!  a little girl who has Bombay blood gets in a car accident and badly needs blood transfusions; the blood transfusions meant for her from a neighboring hospital were stolen by a shady organization for research purposes.
On General Hospital, Monica believed that her son, AJ Quartermaine, could not be her husband Alan's child and must have been the product of her affair with Rick, because AJ's blood type could not have arisen from hers and Alan's.  It then turned out that Alan had the Bombay phenotype, and AJ really was his son.
The Harvard Medical School class of 2009 Second Year Show featured a Hollywood-style dance performance, with dancers representing blood cells inside the body of a character with Bombay-phenotype blood.