## Why "Antigen-antibody complex deposition in tissues, complement activation, neutrophil recruitment, and release of lysosomal enzymes causing tissue damage" is right This is the defining mechanism of Type III hypersensitivity (immune complex-mediated) as illustrated by the structure marked **C**. The clinical presentation—fever, urticaria, arthralgia, and leukocytoclastic vasculitis with fibrinoid necrosis occurring 5–10 days after foreign protein injection—is the classic presentation of serum sickness. Immune complexes deposit in blood vessels, joints, skin, and kidneys; activate complement; recruit neutrophils; and cause tissue damage through release of lysosomal enzymes. This sequence is the hallmark of Type III reactions (Robbins 10e, Ch 5). ## Why each distractor is wrong - **Cross-linking of IgE on mast cells leading to immediate degranulation**: This describes Type I (immediate) hypersensitivity, which presents within minutes to hours with urticaria and anaphylaxis, not the delayed presentation (5–10 days) and vasculitis seen here. - **Antibody binding to cell surface antigens with subsequent complement-mediated or antibody-dependent cellular cytotoxicity**: This is Type II (cytotoxic) hypersensitivity, which targets cell-bound antigens (e.g., hemolytic anemia, Graves' disease) rather than circulating immune complexes depositing in tissues. - **Sensitized T lymphocytes recognizing antigen-presenting cells and releasing cytokines**: This describes Type IV (delayed-type) hypersensitivity, which is T-cell mediated and typically manifests 24–72 hours after antigen exposure (e.g., tuberculin skin test, contact dermatitis), not the immune complex-driven vasculitis seen here. **High-Yield:** Type III hypersensitivity = immune complex deposition in tissues → complement activation → neutrophilic vasculitis with fibrinoid necrosis; classic delayed presentation (5–10 days) distinguishes it from Type I (immediate, minutes) and Type IV (delayed, 24–72 hours). [cite: Robbins and Cotran Pathologic Basis of Disease, 10th Edition, Chapter 5: Inflammation and Repair]
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