ABSTRACT
The first human red cell antigen system to be characterized was the ABO blood group system. Specificity is determined by the terminal sugar in an oligosaccharide backbone structure. The terminal sugars of the oligosaccharides defining groups A and B are immunogenic. In group O, the precursor H oligosaccharide is unaltered. The red cells express either A, B, both A and B, or neither. Antibodies to antigens not expressed in the red cells of a given individual are detected in circulation, as shown in Table 1.
