Carbohydrate-related to blood grouping?

The major blood groups of this system are A, B, AB, and O.O type RBCs lack A or B antigens. These antigens are carbohydrates attached to a precursor backbone, may be found on the cellular membrane either as glycosphingolipids or glycoproteins, and are secreted into plasma and body fluids as glycoproteins. H substance is the immediate precursor on which the A and B antigens are added. This H substance is formed by the addition of fucose to the glycolipid or glycoprotein backbone. The subsequentaddition of N-acetylgalactosamine creates the A antigen, whereas the addition of galactose produces the B antigen.Reference: Harrison&;s Principles of Internal Medicine; 19th edition; Chapter 138e; Transfusion Biology and Therapy