c. The ABO system consists of a series of possible alleles located on chromosome
nine. These multiple alleles (A1, A2, A3, Am, Ax B, Bm, Bx, O and others) at a single
locus constitute a genetic system.
d. Figure 2-1, based on extensive biochemical and genetic data, illustrates the
synthesis of antigens of the ABO and H systems. Similar mechanisms underlie the
assembly of other carbohydrate specificities.
e. In this scheme, the end product of gene action is not an antigen but 1 of a
group of enzymes called glycosyltransferases. These enzymes are not involved in
sugar synthesis but rather in the assembly of sugar chains by the addition of 1 sugar to
another. Each transferase accomplishes the transfer of a specific sugar--the donor
sugar--to another specified sugar--the acceptor sugar. The carbohydrate antigens
reflect the activity of a series of enzymes which, given the raw materials, can act "in
vitro" in the absence of DNA or other genetic material. These resulting carbohydrate
chains, attached to protein (polypeptide) or lipid (fatty acids) provide antigenic specificity
for chain blood groups, for example, ABO, Lewis.
Blood group antigen synthesis.