Hh blood group

Individuals with the rare Bombay phenotype (hh) do not express substance H 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 genotype) as well as to both A and B antigens, and are therefore compatible only with other hh donors.

ABO - Bombay phenotype. It is easily explained by the H enzyme being coded for by a different gene to the A and B alleles.

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 will probably be unable to get it, as no blood bank would have any in stock. Those anticipating the need for blood transfusion (e.g. in scheduled surgery) may bank blood for their own use (i.e. an autologous blood donation) but this option is not available in cases of accidental injury.

Patients who test as type O may have the Bombay phenotype if they have inherited two recessive alleles of the H gene, (their blood group is O_h and their genotype is "hh"), and so do not produce the "H" carbohydrate that is the precursor to the "A" and "B" antigens. It then no longer matters whether the A or B enzymes are present or not, as no A or B antigen can be produced since the precursor antigen is not present.

Despite the designation O, O_h negative is not a sub-group of any other group, not even O negative or O positive. When this blood group was first encountered, it was found not to be of either group A or B and so was thought to be of group O. But on further testing, it did not match even for O negative or O positive because of the absence of antigen 'H'. The H antigen is a precursor to the A and B antigens. For instance, the B allele must be present to produce the B enzyme that modifies the H antigen to become the B antigen. It is the same for the A allele. However, if only recessive alleles for the H antigen are inherited (hh), as in the case above, the H antigen will not be produced. Subsequently, the A and B antigens also will not be produced. The result is an O phenotype by default since a lack of A and B antigens is the O type. The blood phenotype was first discovered in Bombay, now known as Mumbai, in India.