The specificity of the H antigen is determined by the sequence of oligosaccharides. More specifically, the minimum requirement for H antigenicity is the terminal disaccharide fucose-galactose, where the fucose has an alpha(1-2)linkage. This antigen is produced by a specific fucosyl transferase (Galactoside 2-alpha-L-fucosyltransferase 2) that catalyzes the final step in the synthesis of the molecule. Depending upon a person's ABO blood type, the H antigen is converted into either the A antigen, B antigen, or both. If a person has group O blood, the H antigen remains unmodified. Therefore, the H antigen is present more in blood type O and less in blood type AB.