Frequency: This is an extremely rare ABO group which derives the name "Bombay" because it was first discovered to exist among some people living in the region of Bombay, India. Although the group is more likely to occur in East Indians, it is a very rare group even among this population. Also, it is not restricted to East Indians and has been found to exist in Caucasians, Negroes, Japanese, etc.
Inheritance: The Bombay group (Oh) results from the inheritance of two rare recessive h genes which occur at a locus other than the ABO gene locus. Because the h gene is very rare, Bombays often result from consanguineous matings in which parents are blood relatives (e.g., first cousins). Whenever inbreeding occurs, the proportion of rare homozygotes increases in frequency (Figure 4-4).
Reaction Pattern: Bombay groups type as group O in routine ABO grouping as shown below:
|anti-A,B||anti-A||anti-B||A1 cells||B cells|
Their red cells lack ABH antigens and their sera contain anti-A and anti-B and anti-H. The anti-H would not be detected in the ABO group but would be detectable in pretransfusion tests, e.g., their sera would agglutinate group O screen cells and group O donor cells, which have the H antigen.
Significance in blood transfusion: Bombay people would be incompatible when crossmatched with red cells of all normal ABO groups (groups O, A, B and AB).
If they require blood transfusion, they must receive blood from another Bombay. Donors must be sought among their blood relatives (especially siblings) or from the rare donor file maintained by the Red Cross.