Hh antigen system
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Problems with blood transfusion 
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.
This very rare phenotype is generally present in about 0.0004% (about 4 per million) of the human population, though in some places such as Mumbai (formerly Bombay) locals can have occurrences in as much as 0.01% (1 in 10,000) of inhabitants and 1 in a million people in Europe. 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 may bank blood for their own use, but of course this option is not available in cases of accidental injury since one can not always plan for injury.
The biosynthesis of the H antigen and the A and B antigens involves a series of enzymes (glycosil transferases) that transfer monosaccharides. The resulting antigens are oligosaccharide chains, which are attached to lipids and proteins that are anchored in the RBC membrane. The function of the H antigen, apart from being an intermediate substrate in the synthesis of ABO blood group antigens, is not known although it may be involved in cell adhesion. Fortunately people who lack of the H antigen do not suffer from any deleterious effects, and being H-deficient is only an issue if they were to need a blood transfusion because they would require H-deficient blood.
The specificity of the H antigen is determined by the sequence of oligosaccharides. More specifically, the minimum requirement for H antigenicity is the terminal disaccharide Fucose-Galactose, where the fucose has an alpha(1-2)linkage. This antigen is produced by a specific fucosyl transferase that catalyzes the final step in the synthesis of the molecule. Depending upon a person's ABO blood type, the H antigen is converted into either the A antigen, B antigen, or both. If a person has group O blood, the H antigen remains unmodified. Therefore, the H antigen is present more in blood type O and less in blood type AB.
Two regions of the genome encode two enzymes with very similar substrate specificities: the H locus (FUT1) which encodes the Fucosyl transferase and the Se locus (FUT2) that instead indirectly encodes a soluble form of the H antigen, which is found in bodily secretions. Both genes are located on chromosome 19 at q.13.3. - FUT1 and FUT2 are tightly linked, being only 35 kb apart. Because they are highly homologous, they are likely to have been the result of a gene duplication of a common gene ancestor. The H locus contains four exones that span more than 8 kb of genomic DNA. Both the Bombay and para-Bombay phenotypes are the result of point mutations in the FUT1 gene. At least one functioning copy of FUT1 needs to be present (H/H or H/h) for the H antigen to be produced on RBCs. If both copies of FUT1 are inactive (h/h), the Bombay phenotype results. The classical Bombay phenotype is caused by a Tyr316Ter mutation in the coding region of FUT1. The mutation introduces a stop codon, leading to a truncated enzyme that lacks 50 aminoacids at the C-terminal end, rendering the enzyme inactive. In Caucasians, the Bombay phenotype may be caused by a number of mutations. Likewise, a number of mutations have been reported to underlie the para-Bombay phenotype. The Se locus contains the FUT2 gene, which is expressed in secretory glands. Individuals who are "secretors" (Se/Se or Se/se) contain at least one copy of a functioning enzyme. They produce a soluble form of H antigen that is found in saliva and other bodily fluids. "Non-secretors" (se/se) do not produce soluble H antigen. The enzyme encoded by FUT2 is also involved in the synthesis of antigens of the Lewis blood group.
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 they 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 neither A nor 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. 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 families, which are inbred due to custom rather than local genetic variety.
Hemolytic disease of the newborn 
In theory, the maternal production of anti-H during pregnancy might cause hemolytic disease in a fetus who did not inherit the mother's Bombay phenotype. In practice, cases of HDN caused in this way have not been described. This may be possible due to the rarity of the Bombay phenotype but also because of the IgM produced by the immune system of the mother. Since IgMs are too heavy to cross the placental barrier (like indeed the IgG do) they cannot reach the blood stream of the fetus provoking an acute hemolytic reaction.
Popular culture 
- In the horror manga Reiko the Zombie Shop, the character Midori Yurikawa, a friendly, but temperamental young girl who becomes homicidal when assaulted and her more malicious older sister Saki Yurikawa (who died in volume one and came back as a zombie) is revealed to be a carrier of the Bombay blood type and is searched for by Reiko after Dr. Akiyama (the doctor who watched over Midori) committed suicide by slitting her wrists.
- In the cooking anime Yakitate!! Japan, Pierrot was revealed to have Bombay blood, and so was the King of Monaco, who was related to him.
- In the detective anime Get Backers, the protagonists Ban and Ginji are sent on a mission to retrieve Bombay blood for a terminally ill girl named Yumiko. It is later revealed that one of the antagonists possesses the bag with the said blood type and they are forced to fight them to acquire the blood.
- On the daytime soap opera General Hospital, it was believed that Monica's husband Alan could not have been the father of her child, as Alan's blood was AB, Monica's was A, and the child's was thought to be O. However, it was eventually revealed that Alan was indeed the father; he and Monica were both recessive carriers of the h gene and the baby had the Bombay phenotype.
- In the 2007 Telugu film Okkadunnadu, both the protagonist Kiran (Gopichand) and the antagonist mafia don Sonu Bhai (Mahesh Manjrekar) are the only two carriers of the Bombay blood type. Kiran's heart is sought after by Sonu Bhai's henchmen for an urgent heart transplant required by Sonu Bhai.
- In the 2012 Japanese television drama Seinaru Kaibutsutachi, Mie Arima, who has Bombay blood, is killed by the chief nurse Yuka Kasugai with a 0+ blood transfusion while undergoing a Caesarean section.
- In the 2011 Canadian TV series My Babysitter's a Vampire, Ethan, one of the main characters, is said to have H-deficient blood which the vampires of the story consider a delicacy.
- In the 2012 film Get the Gringo the villain (Javi) also has Bombay Blood Group.
- In the 2012 Hindi film Kahaani the main antagonist (Milan Damji) has Bombay Blood Group and this fact was one of the evidence in the thriller film.
- In the new series of the manga Kindaichi Case Files, and later in the special anime episode, the case titled Legend of the Vampire had several characters with the blood type called Bombay.
- Hh at BGMUT Blood Group Antigen Gene Mutation Database at NCBI, NIH
- RMIT University The Bombay, para-Bombay and other H deficiencies