Serum sickness is an example of which type of hypersensitivity reaction?

• A. Type I
• B. Type II
• C. Type IV
• D. Type III

Answer: (D)

Serum sickness is an immune complex–mediated hypersensitivity, a Type III reaction. When a person is exposed to a foreign protein or monoclonal antibody, soluble antigen–antibody complexes form in the circulation and deposit in tissues such as small vessels and the kidneys. This deposition activates complement and recruits inflammatory cells, leading to symptoms like fever, urticarial rash, arthralgias, and sometimes nephritis, usually occurring about 1–2 weeks after exposure. It differs from other hypersensitivity types because it centers on immune complex deposition rather than IgE-mediated mast cell degranulation (Type I), antibody-directed cytotoxicity against cells (Type II), or T-cell–mediated delayed responses (Type IV).