Pathophysiology
Clinical meaning
Blood typing is based on the presence or absence of specific antigens on the surface of red blood cells (erythrocytes) and corresponding antibodies in the plasma. The ABO blood group system is the most clinically significant because ABO antibodies are naturally occurring (present without prior sensitization) and can cause fatal intravascular hemolysis if incompatible blood is transfused. The ABO system is determined by carbohydrate antigens attached to glycoproteins and glycolipids on the red blood cell membrane. The H antigen is the precursor molecule present on all red blood cells. The A gene encodes an enzyme (N-acetylgalactosamine transferase) that adds N-acetylgalactosamine to the H antigen, creating the A antigen. The B gene encodes an enzyme (galactose transferase) that adds galactose to the H antigen, creating the B antigen. Individuals with type O blood have neither A nor B transferase enzymes, so the H antigen remains unmodified. The critical immunological principle is that individuals develop antibodies against ABO antigens they LACK: type A individuals have anti-B antibodies (IgM), type B individuals have anti-A antibodies, type AB individuals have neither antibody (universal plasma...