Which statement best differentiates Type III and Type IV hypersensitivity?

Which statement best differentiates Type III and Type IV hypersensitivity?

• A. Type III depends on immune complexes; Type IV is T-cell mediated
• B. Type III is T-cell mediated; Type IV depends on immune complexes
• C. Both involve antibodies and complement
• D. Type III causes immediate reactions; Type IV is delayed

Answer: (A)

The key difference lies in the underlying effector mechanism. Type III hypersensitivity is driven by immune complexes formed between antibodies and soluble antigens that circulate and deposit in tissues, where they activate complement and recruit neutrophils to cause inflammation and tissue damage. Type IV hypersensitivity is a T-cell–mediated process: sensitized T lymphocytes recognize antigen, release cytokines, and recruit/activate macrophages or directly kill target cells, leading to tissue injury. This is a delayed reaction, typically appearing 24–72 hours after exposure, as seen in contact dermatitis or a tuberculin skin test reaction. The statement that Type III depends on immune complexes and Type IV is T-cell mediated captures this fundamental difference. The other options misstate the roles of antibodies/complement or the timing, since Type IV is not antibody- and complement-driven, and Type III is not an immediate reaction.

The key difference lies in the underlying effector mechanism. Type III hypersensitivity is driven by immune complexes formed between antibodies and soluble antigens that circulate and deposit in tissues, where they activate complement and recruit neutrophils to cause inflammation and tissue damage. Type IV hypersensitivity is a T-cell–mediated process: sensitized T lymphocytes recognize antigen, release cytokines, and recruit/activate macrophages or directly kill target cells, leading to tissue injury. This is a delayed reaction, typically appearing 24–72 hours after exposure, as seen in contact dermatitis or a tuberculin skin test reaction. The statement that Type III depends on immune complexes and Type IV is T-cell mediated captures this fundamental difference. The other options misstate the roles of antibodies/complement or the timing, since Type IV is not antibody- and complement-driven, and Type III is not an immediate reaction.