The H antigen is a precursor to the ABO blood group antigens, present in people of all common blood types. The Bombay phenotype (hh) does not express antigen H on red blood cells, and therefore this type will also lack A or B antigens, similar to the O blood group. However, unlike O group, the H antigen is absent, hence the individuals produce isoantibodies to antigen H as well as to both A and B antigens. If they receive blood from someone with O blood group, the anti-H antibodies will bind to the H antigen on the red blood cells of the donor blood and destroy the RBCs by complement-mediated lysis. Therefore, people with Bombay phenotype can receive blood only from other hh donors, although they can donate as though they were type O. Some individuals with the blood group A1 may also be able to produce anti-H antibodies due to the complete conversion of all the H antigen to A1 antigen. Production of the H antigen, or its deficiency in the Bombay phenotype, is controlled at the H locus on chromosome 19. The H locus contains three exons that span more than 5 kb of genomic DNA, and encodes the fucosyltransferase that produces the H antigen on RBCs. The H antigen is a carbohydrate sequence with carbohydrates linked mainly to protein (with a minor fraction attached to ceramide moiety).
ABH; BG-H; BG-H1; Blood Group H ab antigen