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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. This blood phenotype was first discovered in Bombay, now known as Mumbai, in India, by Dr. Y.M. Bhende, as published in 1952. It is present in about 0.0004% (about 4 per million) of the human population generally, though in some places such as Mumbai (formerly Bombay) local populations can have occurrences in as much as 0.01% (1 in 10,000) of inhabitants. Bombay phenotype ( Bombay Blood group ) is a rare blood group which shows absence of A,B,H antigens on red cells and presence of anti-A, anti-B and anti-H antibodies in serum. The rare individuals with Bombay phenotype do not express H substance on their red blood cells and therefore do not bind A or B antigens. Instead, they produce antibodies to H substance (which is present on all red cells except those of hh phenotype) as well as to both A and B antigens and are therefore compatible only with other hh donors. Genetics ABO - Bombay phenotype. It is easily explained by the H enzyme being coded for by a different gene to the A and B alleles. 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 families, which are inbred due to custom rather than local genetic variety. ABO antigens Diagram showing the carbohydrate chains that determine the ABO blood group The H antigen is an essential precursor to the ABO blood group antigens. The H locus is located on chromosome 19. It contains 3 exons that span more than 5 kb of genomic DNA, and it encodes a fucosyltransferase that produces the H antigen on RBCs. The H antigen is a carbohydrate sequence with carbohydrates linked mainly to protein (with a minor fraction attached to ceramide moiety). It consists of a chain of β-D- galactose, β-D-N-acetylglucosamine, β-D-galactose, and 2- linked, α-L-fucose, the chain being attached to the protein or ceramide. The ABO locus is located on chromosome 9. It contains 7 exons that span more than 18 kb of genomic DNA. Exon 7 is the largest and contains most of the coding sequence. The ABO locus has three main alleleic forms: A, B, and O. The A allele encodes a glycosyltransferase that bonds α-N-acetylgalactosamine to D- galactose end of H antigen, producing the A antigen. The B allele encodes a glycosyltransferase that joins α-D-galactose bonded to D-galactose end of H antigen, creating the B antigen. In case of O allele, the exon 6 contains a deletion that results in a loss of enzymatic activity. The O allele differs from the A allele by deletion of only one nucleotide – guanine at position 261. The deletion causes a frameshift, and results in premature termination of translation, and thus, degradation of the mRNA. This results in H antigen remaining unchanged in case of O groups. The majority of the ABO antigens are expressed on the ends of long polylactosamine chains attached mainly to band 3 protein, the anion exchange protein of the RBC membrane, and a minority of the epitopes are expressed on neutral glycosphingolipids.
"hh is a rare blood group also called Bombay Blood group"