The Bombay Phenotype

The discovery of the ABO blood group, over 100 years ago, caused great excitement. Until then, all blood had been assumed to be the same, and the often tragic consequences of blood transfusions were not understood. As our understanding of the ABO group grew, the world of blood transfusion became a great deal safer. The four basic ABO phenotypes dicovered by Karl Landsteiner in 1901 were O, A, B, and AB.

Bombay Blood Group

The discovery of Bombay Blood Group was made in 1952 by Dr.Y.M. Bhende in Mumbai with a woman who was admitted to KEM Hospital and required blood transfusions. In need of transfusion, she was found to lack both the A and B antigens and was thus typed as ‘O’. Later it was discovered to have an interesting blood type that reacted to other blood types in a way that had not been seen before. Serum from this individual contained antibodies that reacted with all Red Blood Cells (RBCs) from normal ABO phenotypes (i.e., groups O, A, B, and AB). The woman's RBCs appeared to lack all of the ABO blood group antigens plus an additional antigen. This new blood group character was the H antigen. It is now known that the precursor protein from which all blood groups are formed is termed as the ‘H’ Antigen. The ‘H’ Antigen either translates into ‘A’ Antigen (the blood group is then called ‘A’) or it translates into ‘B’ Antigen (the blood group is then called ‘B’) or it translates into both ‘A’ and ‘B’ Antigens (the blood group is then called ‘AB’) or it remains as ‘H’ Antigen (the blood group is called ‘O’).  Earlier the detection of ‘O’ was on the basis of absence of both ‘A’ and ‘B’.

In the case of Bombay Blood Group, a rare genetic trait involving the phenotypic expression of the ABO blood groups, there was an absence of the ‘H’ Antigen itself.  Therefore ‘A’, ‘B’, ‘AB’ and ‘O’ which are all different manifestations of ‘H’ are all exotic for persons with Bombay Blood Group.  The Bombay Blood Group is termed as ‘OH’(h/h genotype), which means absence of ‘H’. Because the A and B antigens cannot be formed without the H antigen precursor, their RBCs also lack these antigens. As a result, these individuals produce anti-H, anti-A, and anti-B and can therefore be transfused only with RBCs that also lacks the H, A, and B antigens i.e., they can only receive blood from another person with the Bombay phenotype.

Detection Of Bombay Phenotype

This blood group may be commonly mistaken as 'O' and many a time, not identified at all if proper blood grouping or testing practices are not followed. During routine grouping conducted at the hospital's blood bank, the blood group may show up as O, with no reaction to Anti-A and Anti-B antibodies. However, Bombay Phenotype is detected when reverse grouping is performed, it shows agglutination with O cells, proving the rare Bombay 'Oh' Phenotype blood. It is important to be cautious in predicting the ABO blood type of children based on the phenotypes of their parents.  This is due to the fact that a third antigen (H) on the surface of red cells can prevent the expected ABO blood type from occurring. Repeated testing are then done as it is important to perform reverse grouping or serum grouping to detect the Bombay Blood group.

Antibody Production & Clinical Significance of H antibodies

If patients with anti-H in their circulation (as in Bombay blood group) receive transfusions of blood that contains the H antigen (e.g., blood group O), they are at risk of suffering an acute haemolytic transfusion reaction. The maternal production of anti-H during pregnancy could cause haemolytic disease in a foetus who did not inherit the mother's Bombay phenotype. However, cases of Haemolytic disease of the new-born caused in this way have not been reported, possibly because of the rarity of the Bombay phenotype.

Frequency- A Rare Blood Type:

Bombay Phenotype is one of the rarest blood groups: Approximately one in every 17,600 people in India or one in every 25,000 people in the world has this group. In some places such as Mumbai locals can have occurrences in as much as 0.01% (1 in 10,000) of inhabitants. The Bombay phenotypes were also detected in Japan, Malaysia, Thailand, and Srilanka indicating that it is mostly confined to South East Asian countries.

Since this condition is very rare if a blood transfusion is ever needed, people with this blood type can receive blood only from other donors who are also H deficient. At times when this rare blood group is required for surgeries, blood from blood banks in other cities has to be brought, or donors from other locations have to travel to the hospital where blood is required as it would be quite unlikely that any blood bank would have any in stock.

Geneticists believe that the high number of Bombay blood group people in India is the result of consanguineous marriage among members of a caste class. Higher caste class allows consanguineous marriage to maintain their position in the society and to protect their wealth.

(written by- Sanchit Agarwal- HansRaj College)

 

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