Which type of hypersensitivity is driven by immune complex deposition in tissues?

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

Answer: (C)

Immune complex–mediated tissue injury is the hallmark of Type III hypersensitivity. When soluble antigen–antibody complexes form, they can deposit in vessel walls and other tissues. These immune complexes activate the classical complement pathway, generating C3a and C5a, which recruit and activate neutrophils. The neutrophils release enzymes and reactive oxygen species that damage surrounding tissue, producing inflammation and injury seen in conditions like serum sickness, Arthus reactions, and some forms of glomerulonephritis. This differs from the other hypersensitivity types: Type I is IgE-mediated mast cell degranulation; Type II involves antibodies against cell surface or extracellular matrix leading to cytotoxicity or opsonization; Type IV is T-cell–mediated delayed-type hypersensitivity.