A 54-year-old woman is involved in an automobile accident and requires a blood transfusion. Five hours later, she becomes febrile and has severe back pain. Laboratory studies show evidence of intravascular hemolysis. It is discovered that type A Rh+ blood was given by mistake to this type B Rh+ patient. Which of the following best explains the development of intravascular hemolysis in this patient?

Antibody-mediated complement fixation. Type II hypersensitivity reactions are mediated by antibodies directed against fixed antigens. In this case, preformed antibodies in the patient's blood attached to foreign antigens (oligosaccharides) on the membranes of the transfused erythrocytes. At sufficient density, bound immunoglobulins fix complement. Once activated, the complement cascade leads to the destruction of the target cell through formation of a membrane attack complex. This type of complement-mediated cell lysis occurs in autoimmune hemolytic anemia. Antibody-dependent cell-mediated cytotoxicity (ADCC, choice A) involves cytolytic leukocytes that attack antibody-coated target cells. ADCC may be involved in the pathogenesis of some autoimmune diseases (e.g., autoimmune thyroiditis). Delayed-type hypersensitivity (choice C) occurs over a period of days and does not involve preformed antibodies.Diagnosis: Hemolytic anemia, jaundice