Hh blood group

Rare blood type

hh, or the Bombay blood group, is a rare blood type. This blood phenotype was first discovered in Bombay by Dr. Y. M. Bhende in 1952. It is mostly found in the Indian sub continent (India, Bangladesh, Pakistan) and Iran.

The first person found to have the Bombay phenotype had a blood type that reacted to other blood types in a way never seen before. The serum contained antibodies that attacked all red blood cells of normal ABO phenotypes. The red blood cells appeared to lack all of the ABO blood group antigens and to have an additional antigen that was previously unknown. In 2023, it was reported that only three registered Brazilians nationwide possessed this phenotype.

Biosynthesis of the H, A and B antigens involves a series of enzymes (glycosyl transferases) that transfer monosaccharides. The resulting antigens are oligosaccharide chains, which are attached to lipids and proteins that are anchored in the red blood cell 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. People who lack the H antigen do not suffer from deleterious effects, and being H deficient is only an issue if they need a blood transfusion, because they would need blood without the H antigen present on red blood cells. 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 (Galactoside 2 alpha L fucosyltransferase 2) 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 transferase, which is found in bodily secretions. Both genes are 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 exons 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 red blood cells. 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 amino acids 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.

Bombay phenotype occurs in individuals who have inherited two recessive alleles of the H gene (i. e. their genotype is hh). These individuals do not produce the H carbohydrate that is the precursor to the A and B antigens, meaning that individuals may possess alleles for either or both of the A and B alleles without being able to express them. 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 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.

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 not transported across the microscopic placental blood vessels (like IgG are) they cannot reach the blood stream of the fetus to provoke the expected acute hemolytic reaction.