Clinical meaning
Blood transfusion reactions occur when the recipient's immune system recognizes transfused blood components as foreign and mounts an immune response. Understanding the immunological basis of transfusion reactions begins with the ABO and Rh antigen systems present on red blood cell surfaces. The ABO system is based on carbohydrate antigens: type A blood has A antigens with anti-B antibodies in plasma, type B has B antigens with anti-A antibodies, type AB has both antigens with neither antibody (universal plasma recipient), and type O has neither antigen with both antibodies (universal red cell donor). The Rh system involves the D antigen -- Rh-positive individuals have the D antigen, while Rh-negative individuals lack it and can develop anti-D antibodies after exposure. Acute hemolytic transfusion reactions (AHTR) are the most severe type, occurring when ABO-incompatible blood is transfused. The recipient's pre-existing IgM antibodies bind to incompatible donor red blood cells, activating the complement cascade (classical pathway). Complement activation leads to formation of the membrane attack complex (C5b-9) which creates pores in donor RBC membranes, causing intravascular hemolysis. The released free hemoglobin overwhelms haptoglobin binding...