the Bombay blood type is very rare

hh is a rare blood group also called Bombay Blood group. Individuals with the rare Bombay phenotype (hh) do not express H antigen (also called "substance H") (the antigen which is present in blood group O). As a result, they cannot make A antigen (also called "substance A") or B antigen (also called "substance B") on their red blood cells, whatever alleles they may have of the A and B blood-group genes, because A antigen and B antigen are made from H antigen; receiving blood which contains an antigen which has never been in the patient's own blood causes an immune reaction. As a result, people who have Bombay phenotype can donate to any member of the ABO blood group system (unless some other blood factor  gene, such as Rhesus, is incompatible), but they cannot receive any member of the ABO blood group system's blood (which always contains one or more of A and B and H antigens), but only from other people who have Bombay phenotype. The usual tests for ABO blood group system would show them as group O, unless the hospital worker involved has the means and the thought to test for Bombay group.

This blood phenotype was first discovered in Bombay, now known as Mumbai, in India, by Dr. Y.M. Bhende.[1] It is present in about .0004% of the human population generally, though in some places such as Mumbai (formerly Bombay) local populations can have occurrences in as much as .01% of inhabitants.

The H antigen is located on the surface of red blood cells and is the precursor of A and B antigens. The A allele is needed to produce a transferase enzyme to modify the H antigen into A antigen. Likewise, the B allele is needed to make the transferase enzyme that would transform the H antigen into B antigen. For type O individuals, the H antigen cannot be transformed further because no functional transferase enzyme is produced to modify the antigen. Note that antigen modification is through the addition of complex carbohydrates into the H antigen by the transferase enzyme.

People of the Bombay
blood group produce antibodies against H, A, and B antigens to protect themselves. Since they have antibodies against H, A, and B antigens, they can only receive blood donations from other people with Bombay blood type. Receiving blood transfusions from the ABO blood group can be fatal. The antibodies of the Bombay blood react to the red blood cells of the donor and causes cell death. In the past, some patients who were classified as type O by the ABO test died because doctors failed to test them for the Bombay blood type.

Because the Bombay blood type is very rare, it is very hard to find blood donors for patients who need it. The probability of finding a person with Bombay blood type is 1 for every 250,000 people. India has the highest number of people with the Bombay blood group where there is one Bombay blood type per 7,600 people. 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.

 Bombay Blood Group

A person of the Bombay blood group inherited the recessive form of the allele for the H antigen from each of his parents. He carries the homozygous recessive (hh) genotype instead of the homozygous dominant (HH) or heterozygous (Hh) genotypes of the ABO blood group. As a result, the H antigen is not expressed in the red blood cell surfaces; consequently, the A and B antigens are not formed. The h allele is a result of the mutation of the H gene (FUT1) that would express the H antigen in the red blood cells of ABO blood group. Scientists found that people of the Bombay phenotype are homozygous (hh) for the T725G mutation (Leucine 242 is changed to Arginine) in the FUT1 coding region. The consequence of this mutation is the production of an inactivated enzyme that is incapable of producing the H antigen.