The Bombay blood group

The Bombay blood group

The Bombay phenotype — also written hh or Oh — is a rare blood type whose red cells carry no A, B or H antigen. It matters because someone who has it can normally only receive blood from another person who has it, and there are very few of them.

What it is

Most blood grouping is about two markers, A and B. Both are built on a third one, the H antigen, which acts as the foundation the others are added to. People with the Bombay phenotype cannot make the H antigen at all, so A and B never get built either.

On a routine test this looks like group O, because none of the usual markers are found. It is not group O. A person with the Bombay phenotype also carries anti-H in their plasma — an antibody against the H antigen that almost everyone else’s red cells display.

How rare it is

In India the Bombay phenotype is usually given as roughly 1 in 10,000 people. Screening studies put it lower still. A study of 26,638 people in Andhra Pradesh found 13 cases (0.048%). A study of 28,934 blood donors in southern West Bengal found two.

It is not spread evenly. Prevalence is markedly higher in some communities — reported at around 1 in 278 among the Bhuyan tribal population of north-western Orissa — which is what makes population-level averages misleading for any individual family.

For comparison: a blood bank will hold O-negative on the shelf as a matter of routine. It will almost certainly never have held a unit of Bombay blood.

Why it runs in families

The phenotype is inherited in an autosomal recessive pattern: a person has it only if they inherit a non-working copy of the gene from both parents. Parents who each carry one working and one non-working copy have the ordinary blood group themselves and no reason to suspect anything.

Two genes are involved, FUT1 and FUT2, sitting close together on chromosome 19. FUT1 governs the H antigen on red cells; FUT2 governs it in saliva and other secretions. The Bombay phenotype involves non-working versions of both. Where only FUT1 is affected, the result is the related para-Bombay phenotype.

Because it is recessive, the practical consequence is simple: siblings are the most likely match, and consanguineous marriage raises the chance of a child having it. When someone is identified, testing close relatives is usually the fastest route to finding a compatible donor.

Why ordinary O-negative is not a substitute

O-negative is described as the universal donor, and for almost everyone it is. The Bombay phenotype is the exception that breaks the rule.

O-negative red cells still carry the H antigen. A recipient with the Bombay phenotype has anti-H in their plasma, which attacks those cells. Giving O-negative to someone with the Bombay phenotype risks a serious haemolytic transfusion reaction. Only blood from another person with the Bombay phenotype is safe — or the patient’s own blood, stored in advance where that is possible.

This is also why the phenotype is dangerous to miss. It reads as group O on a standard forward test, and is identified by the reverse test and by specific anti-H testing.

Why a register helps

Nothing above can be changed. What can be changed is the search. Today, when a hospital needs Bombay blood, it starts from phone calls, word of mouth and social media appeals — often while a patient waits. The compatible donors usually exist. Nobody has a list of them.

A register does not make rare blood less rare. It makes the people who have it findable, which is the part that can be fixed.

Join the register

Sources