Which of the following is an example of a type II hypersensitivity reaction in which antibodies against cell-surface antigens activate complement causing cell damage?

• A. ABO transfusion reaction
• B. Allergic reaction to penicillin
• C. Serum sickness reaction
• D. Skin reaction to poison ivy

Answer: (A)

Type II hypersensitivity is antibody-mediated cytotoxicity, where antibodies target cell-surface antigens and activate complement to destroy the cells. An ABO transfusion reaction is the classic example: preformed anti-ABO antibodies (primarily IgM) bind to A or B antigens on donor red blood cells, fix the classical complement pathway, and cause intravascular hemolysis and cell damage. The other options involve different mechanisms—penicillin allergy is IgE-mediated (type I), serum sickness is immune complex–mediated (type III), and poison ivy skin reaction is delayed-type cell-mediated (type IV).