Which condition exemplifies a Type III hypersensitivity reaction?

• A. Anaphylaxis
• B. Transfusion reaction
• C. Contact dermatitis
• D. Serum sickness, SLE, RA

Answer: (D)

Type III hypersensitivity is driven by immune complexes—antigen–antibody complexes formed in circulation that deposit in tissues such as blood vessels, kidneys, and joints. Once deposited, they activate complement and recruit neutrophils, causing inflammation and tissue damage. Serum sickness is a classic example where soluble antigen–antibody complexes trigger this process; systemic lupus erythematosus and rheumatoid arthritis also involve immune complex deposition in tissues, leading to inflammation in areas like the kidneys and joints. By contrast, anaphylaxis is mediated by IgE and mast cell degranulation (Type I), and contact dermatitis is driven by sensitized T cells (Type IV). Transfusion reactions can involve different mechanisms, but aren’t the archetypal immune complex–mediated responses seen in these conditions. So the listed condition set best illustrates immune complex–mediated tissue injury characteristic of Type III hypersensitivity.