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hh is a rare blood group also called Bombay Blood group

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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.[1] 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.[citation needed]
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.

				
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