The lack of A, B, or H antigens on red blood cells and in secretions and plasma. As outlined here, the H antigen is made by adding a fucose sugar to an oligosaccharide, and that must happen before either A or B antigens can be made (i.e., if you can’t make H, you can’t make A or B!). Bombay individuals lack the two genes that encode the enzymes to make H antigen, H and Se (i.e., their genotype is hh and sese), so H is not made on RBCs or in secretions/plasma. Bombay persons’ serum contains very strong anti-H and anti-A, -B, or -A,B; this essentially renders their antibodies incompatible with all other RBCs except those from another Bombay individual. Bombay and the similar-sounding Para-Bombay phenotype are closely related. Watch my 2014 BBGuy video on Bombay for more information.