Blood transfusion reactions result from recipient immune response to foreign antigens on donor blood components. Acute hemolytic transfusion reactions (AHTR) are the most dangerous, caused by ABO incompatibility where pre-formed IgM antibodies bind donor red blood cells, activating complement cascade leading to intravascular hemolysis, hemoglobinemia, hemoglobinuria, DIC, and renal failure. Febrile non-hemolytic reactions are the most common, caused by recipient antibodies reacting to donor white blood cell antigens or cytokines accumulated during storage. Transfusion-related acute lung injury (TRALI) involves donor antibodies activating recipient neutrophils in pulmonary capillaries, causing non-cardiogenic pulmonary edema. Transfusion-associated circulatory overload (TACO) results from volume overload, particularly in patients with compromised cardiac function.