ABO Blood Group System

          The ABO blood group system was the first system described and remains the most significant used for transfusion medicine. A mismatch of ABO may be fatal, whereas a mismatch of other blood groups initially is harmless. This situation occurs because anti-A and anti-B usually are present in the blood of adults lacking the corresponding antigen. These antibodies are stimulated by the ubiquitous distribution of the antigen that forms part of the membrane structure of many bacteria, plants, and animals. For this reason, all donor blood for transfusion is tested and labeled with the ABO group. The four main phenotypes are A, B, AB, and O, the latter indicating a lack of both A and B antigens. The sugars defining A and B antigens are added to carbohydrate chains carrying the H antigen (fucose), which is "hidden" by the A or B sugar. Thus group A or B erythrocytes appear to have less H antigen than group O cells. Nonetheless, H is found on all human erythrocytes except those in rare individuals of the O_h (Bombay) phenotype.