hh antigen blood group system also known as BOMBAY BLOOD GROUP is the rarest blood group in the world found in only 4 persons per 1000000 persons in the world this makes it the ultra rare blood group in the world.
This blood phenotype was first discovered in Bombay now known as Mumbai by Dr. Y. M. Bhende. The individual with blood group can only give or receive blood only from another person who has a similar blood group, hence as a result any person in need of this rare blood group the transfusion would be highly difficult since such blood is not available in the blood banks.
The rare individuals with Bombay phenotype do not express H substance on their red blood cells and therefore do not bind A or B antigens. Instead, they produce antibodies to H substance (which is present on all red cells except those of hh phenotype) as well as to both A and B antigens and are therefore compatible only with other hh donors.
Individuals with Bombay phenotype blood groups can only be transfused with blood from other Bombay phenotype individuals. Given that this condition is very rare to begin with, any person with this blood group, who needs an urgent blood transfusion, may be simply out of luck, as it would be quite unlikely that any blood bank would have any in stock.
If a Bombay Blood Group recipient is not transfused the blood of a Bombay Blood Group person, it can lead to a haemolytic transfusion reaction, which can be fatal and lead to death.