Complement-mediated cytotoxicity is most relevant in which clinical scenario?

• A. Autoimmune thyroid disease
• B. Bacterial sepsis
• C. Osteoporosis
• D. Transfusion reactions

Answer: (D)

Complement-mediated cytotoxicity happens when antibodies bound to a target cell activate the classical complement pathway, forming the membrane attack complex that lyses the cell. In transfusion reactions, recipient antibodies against donor red cell antigens (often ABO) fix complement on donor RBCs, causing rapid intravascular hemolysis and release of hemoglobin. This direct destruction of transfused red cells by complement is the clearest example of this mechanism. Autoimmune thyroid disease involves antibodies affecting thyroid tissue rather than causing widespread RBC destruction via complement. Bacterial sepsis uses complement to help clear infection through opsonization and inflammation rather than causing cytotoxic lysis of host cells. Osteoporosis isn’t driven by antibody- or complement-mediated cytotoxicity. So transfusion reactions are where this mechanism is most relevant.