It is a rare blood group with incidence of 0.0004% of human population
This group was first discovered in Bombay, now known as in India, by Dr. Y. M. Bhende in 1952. It is mostly found in India, Pakistan and Iran .
H antigen is absent in person having Bombay blood group where as H antigen is present in the person with O group.
H antigen is required for the expression of A and B antigens which are glycoproteins
H antigen is produced by the H gene (called FUT1) which is located on chromosome 19 (ABO gene is located on chromosome 9). 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. They inherit two recessive allele of H gene ( genotype hh)
H antigen is converted to their respective glycoproteins depending upon the enzymes ( glycosyl transferases) produced in the person
A allele produces α-1,3-N-acetyl galactosamine transferase which adds N-Acetyl galactosamine to the H- antigen converting into A antigen in both A and AB individuals.
B allele encodes α-1,3-Galactosyl transferase which adds Galactose to H antigen converting into B antigen in both B and AB individuals.
Person with AB blood group has both the enzymes which add both N-Acetyl galactosamine and galactose to the H-anigen
O allelelacks both enzymatic activity and have H antigen.
In the person with Bombay blood group , the red cells are not agglutinated by anti-A or anti-B regardless of ABO genotype and they are also not agglutinated by anti-H
Serum of Bombay blood group patients contains potent anti-A, anti-B and anti-H. This allows only the blood of Bombay group to be transfused to these persons
References
A.Victor Hoffbrand, Douglas R Higgs, David M Keeling, Atul B Mehta. Postgraduate Haematology. 7th edition. 2016.
