A close-up shot of the blood stored in a blood bag hanging in a clinic.
Here is a situation that actually happens in Indian hospitals, more than most people realise.
A patient needs blood urgently. Their sample is tested and typed as O. O-negative units are prepared and transfused. The patient goes into a severe haemolytic reaction — a medical emergency.
What went wrong? The patient didn't have type O blood. They had the hh phenotype — the Bombay blood group. Their cells carried no A antigen, no B antigen, and no H antigen. Routine ABO testing mistook this for type O. But in their plasma sat anti-H antibodies. And O blood — the supposedly "universal" option — is rich in H antigen. The antibodies attacked the transfused cells.
This is not a theoretical scenario. A published Indian study documented three Bombay phenotype patients in a single hospital in central India, two of whom were initially missed during blood grouping — meaning the wrong blood nearly reached them.
Every discussion of the Bombay blood group starts with understanding H antigen, because the Bombay phenotype is defined by its complete absence.
H antigen is a carbohydrate structure on the surface of red blood cells. It is not just one more antigen in a long list — it is the building block from which A and B antigens are made. An enzyme called fucosyltransferase, produced by a gene called FUT1, adds a specific sugar molecule (fucose) to a precursor structure on the red cell surface. The result is H antigen.
Type O blood has abundant H antigen — because O individuals have neither the A-converting enzyme nor the B-converting enzyme, and H antigen is left unconverted.
Type A blood converts some H antigen to A antigen.
Type B blood converts some H antigen to B antigen.
Type AB blood converts H antigen into both A and B.
In all cases, some H antigen remains. All common blood groups carry H antigen at varying levels.
People with the Bombay phenotype carry a mutation in both copies of the FUT1 gene that completely prevents H antigen production. Without H antigen, there is nothing for the A or B converting enzymes to work on. So even if the person carries A or B blood group genes, those genes produce nothing functional on their red cells.
Their cells carry no A, no B, no H antigens.
Their plasma carries anti-A, anti-B, and anti-H antibodies.
That last one — anti-H — is the dangerous one. Anti-H is a powerful antibody that reacts with H antigen. And H antigen is present in every standard blood type, including O. There is no ABO blood type these patients can safely receive. The only safe blood is from another hh individual.
The Bombay blood group affects approximately 1 in 10,000 people in India, compared to around 1 in 1,000,000 in Europe. India accounts for the vast majority of Bombay phenotype patients globally.
In the Mumbai region specifically, the frequency is even higher — approximately 1 in 7,600. Globally, the highest known frequency is on Réunion Island at approximately 1 in 1,500.
Why is India so heavily affected? The main reason is endogamy — the tradition of marriage within the same community, caste, or social group. The hh phenotype requires inheriting two defective copies of the FUT1 gene, one from each parent. When communities have married within themselves for generations, recessive genes circulate more frequently within those gene pools. The same dynamic that raised certain genetic disease rates in specific communities also raised the Bombay phenotype frequency.
Caste and community boundaries, limited gene flow historically, and consanguineous marriages in some regions all contribute to the higher Indian frequency.
High incidence of the Bombay phenotype has been reported in some tribal populations of Odisha — among the Kutia Kondh tribe — and another study from northwestern Odisha reported an average of 1 in 278 Bombay phenotype among the Bhuyan tribal population. That is extraordinary — roughly 100 times the Indian national average.
The Bombay blood group was first discovered in 1952 by Dr. Y.M. Bhende, along with C.K. Deshpande and H.M. Bhatia, in Bombay (now Mumbai).
The discovery happened because a patient's blood behaved strangely. Routine ABO testing gave an O result, but the blood reacted to anti-H antisera in a way O blood should not — it showed no agglutination, meaning the H antigen was absent. The team recognised this as a phenotype distinct from anything previously described and published their findings. The blood group was named after the city where it was found.
It remains one of Indian medicine's contributions to global transfusion science.
The most dangerous aspect of the Bombay phenotype is how easily it is missed.
On routine ABO forward grouping — the test where antibodies are applied to the patient's red cells — hh blood shows no reaction to anti-A or anti-B antisera, because the cells carry neither antigen. The result looks exactly like type O.
The difference only shows up in reverse grouping — where the patient's plasma is tested against known A and B cells — and even more specifically in testing with anti-H antisera (lectin from Ulex europaeus). Bombay plasma reacts strongly with anti-H, showing agglutination. O plasma reacts weakly or not at all.
Many smaller blood banks do not routinely perform anti-H testing. This is a gap. When a Bombay patient shows up in the emergency department of a hospital without this awareness, they can be mistyped, given the wrong blood, and suffer a transfusion reaction that might be fatal before anyone understands what happened.
The published recommendation from Indian transfusion medicine specialists is clear: blood grouping should be done with serious intention, incorporating both forward and reverse grouping, so that no patient receives wrong blood leading to fatal haemolysis.
This is the medical problem that no routine blood bank can solve with standard inventory.
1. Find another Bombay donor. Extraordinarily difficult. India has a handful of registered Bombay donors nationally, typically around 350 in the unofficial registry — and of those, perhaps 30 are immediately available at any given time. Searching this registry requires knowing the patient is Bombay, having access to the registry, and reaching a donor in time.
2. Autologous blood donation. If surgery is planned in advance, the Bombay patient can donate their own blood before the procedure and have it stored for use during surgery. This requires knowing the blood type well in advance.
3. Acute normovolemic haemodilution (ANH). Two of the three Bombay phenotype patients in the central India study were successfully managed with perioperative acute normovolemic haemodilution, a technique where the patient's own blood is diluted and then returned during surgery to reduce the actual blood cell loss.
4. Surgical blood conservation. Cell salvage during surgery, meticulous operative technique to minimise blood loss, and pharmacological haemostatic agents all reduce the volume of transfusion needed.
For emergency situations without advance preparation, options are severely limited. This is why the Bombay blood group registry exists and why maintaining and expanding it is a medical priority.
Most people with Bombay phenotype don't know they have it until routine blood testing returns an unexpected result, a family member's unusual typing is investigated, or they need a transfusion.
Signs that warrant anti-H testing:
If you are confirmed as Bombay phenotype, the immediate priorities are:
The Bombay blood group is genuinely rare. But India has tens of thousands of people with it. And some of them don't know yet.
Understanding it matters not just for those affected, but for every person involved in blood banking in India — because the hh phenotype is the one case where giving O-negative blood to a patient who looks like type O can kill them.
Register on TheBloodApp and indicate your blood type clearly. If you have been identified as having an unusual blood group, contact your nearest blood bank about the Rare Donor Registry. To find blood banks near you or make an urgent blood request, call the number listed in the app.
Sources: PMC — Bombay Blood Group Southern Bengal 2013 | PMC — Detection Bombay Oh Phenotype Central India 2015 | Ananta IAS — Bombay Blood Group Guide | SCTIMST — Bombay Blood Group Chitra Experience 2024 | PharmaEasy — Bombay Blood Group | JournalsofIndia — hh Phenotype | Wikipedia — Bombay Blood Group
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