• The four most common blood groups are A, B, AB and O. 
• The rare, Bombay blood group was first discovered in Mumbai (then Bombay) in 1952 by Dr Y M Bhende.
• Each red blood cell has antigen over its surface, which helps determine which group it belongs to. 
• The Bombay blood group, also called hh, is deficient in expressing antigen H, meaning the RBC has no antigen H.
• For instance, in the AB blood group, both antigens A and B are found. A will have A antigens; B will have B antigens. In hh, there are no A or B antigens.

Rare in India, rarer globally

• Globally, the hh blood type has an incidence of one in four million. It has a higher incidence in South Asia; in India, one in 7,600 to 10,000 are born with this type.
• The blood type is more common in South Asia than anywhere else because of inbreeding and close community marriages. “It is genetically passed,”  Shared common ancestry among Indians, Sri Lankans, Pakistanis and Bangladeshis has led to more cases of hh blood phenotype in this region.

Testing for the group

• To test for hh blood, an Antigen H blood test is required. Often the hh blood group is confused with the O group. The difference is that the O group has Antigen H, while the hh group does not.If anyone lacks Antigen H, it does not mean he or she suffers from poor immunity or may be more prone to diseases. Their counts for haemoglobin, platelets, white blood cells and red blood cells are similar to the count of others based on their health index. Because of rarity, however, they do face problems during blood transfusion.

Transfusion limitations

• A 2015 study in the Asian Journal of Transfusion Science observed: “The individuals with Bombay blood group can only be transfused autologous blood or blood from individuals of Bombay hh phenotype only which is very rare.” Rejection may occur if they receive blood from A, B, AB or O blood group. In contrast, hh blood group can donate their blood to ABO blood types.
• An unofficial registry for Bombay blood group lists over 350 donors across India. “But at any time there are only 30 active donors available, This group is generally not stored in blood banks, mainly because it is rare and the shelf life of blood is 35-42 days. 
• So, whenever there is a demand for a Bombay blood group patient, a donor is required very urgently. Sometimes, facilities need to be created for transporting the donated blood from one city to another. Two weeks ago, a patient in Kota got hh blood from a Pune-based donor. The blood was flown to Jaipur and taken to Kota hospital by road.

What are antigens?

• Antigens are proteins or carbohydrates which our immune system can recognise. Any antigen that is ‘foreign’ to our immune system is destroyed by an antibody.

What are antibodies?

• Antibodies are attack molecules our immune system makes to protect ourselves against foreign things such as bacteria and viruses. Antibodies can also be formed in response to different blood groups. Everyone is born with some antibodies.

Why AB+ is called univarsal recipient?

• People with type AB+ blood are universal recipients because they have no antibodies to A, B or Rh in their blood and can receive red blood cells from a donor of any blood type.

Why O- is called universal donor?

• Type O blood lacks antigens on the outside of the red blood cells. This means that our immune system will not react to the blood because there is no antigen to react against.
•  Similarly, Rh negative blood lacks the Rh antigens on the outside of the blood cells, so once again there are no proteins for our immune system to react against. This lack of antigens means that type O negative blood can be transfused in people of any blood type; hence, the term universal blood donor.
• Red blood cells from a donor that is type O+ can be transfused into patients of four different blood types: A+, B+, AB+, and of course O+.