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 capacity, leading to hemoglobinuria, renal tubular damage, and potentially acute kidney injury. Simultaneously, complement activation triggers mast cell degranulation, releasing histamine, and activates the coagulation cascade, potentially causing disseminated intravascular coagulation (DIC). Febrile non-hemolytic transfusion reactions (FNHTR) are the most common type, caused by recipient antibodies reacting with donor leukocyte antigens or by cytokines (IL-1, IL-6, TNF-alpha) that accumulate in stored blood products. These cytokines act on the hypothalamic thermoregulatory center, causing fever. Allergic transfusion reactions range from mild urticarial reactions (IgE-mediated mast cell degranulation causing histamine release) to severe anaphylaxis (massive histamine release causing vasodilation, bronchospasm, and cardiovascular collapse). Transfusion-associated circulatory overload (TACO) occurs when transfusion rate exceeds the patient's cardiac compensatory capacity, causing hydrostatic pulmonary edema. Transfusion-related acute lung injury (TRALI) involves donor antibodies reacting with recipient neutrophils in the pulmonary vasculature, causing neutrophil activation, endothelial damage, and non-cardiogenic pulmonary edema within 6 hours of transfusion. The practical nurse must recognize the distinct presentations of each reaction type because the interventions differ significantly, and delays in recognition can be fatal.