hh is a rare blood group also called Bombay Blood group. This blood phenotype was first discovered in Bombay, now known as Mumbay, in India, by Dr. Y.M. Bhende in 1952. This person was discovered 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 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.
Receiving blood which contains an antigen which has never been in the patient's own blood causes an immune reaction due to the reason that the immunity system of an hypothetic receiver produces immunoglobulins against antigen A and B but also against H antigen. The most common immunoglobulins synthesized are the IgM despite of the IgG (and this seems to have a very important role on the low hemolytic disease of the newborn as we are going to see in the last part of this treaty).
It's very important in order to avoid any complications during a blood transfusion to detect a Bombay phenotype carriers because the usual tests for ABO blood group system would show them as group O. Since Anti-H immunoglobulins can activate the complement cascade which lysis 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.
So, individuals with Bombay phenotype blood group can only be transfused with blood from other Bombay phenotype individuals. 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 the preservation of blood doesn't last more than 40 days.
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 does 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.
Hh antigen system - diagram showing the molecular structure of the ABO antigen system
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. When Bombay blood group was first encountered, it was found not to contain antigens A or B and so it 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 of 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 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 immunity system of the mother. Since IgMs are too heavy to cross the ematoplacentar barrier (like indeed the IgG do) they cannot reach the blood stream of the fetus provoking an acute hemolytic reaction.