A 35-year-old man with a history of rheumatoid arthritis develops a maculopapular rash, fever, and joint pain 7 days after starting a new antibiotic. Laboratory studies show elevated ESR, circulating immune complexes, and low serum complement levels. Which of the following is the most common organ system affected by Type III hypersensitivity (immune complex-mediated) disease in clinical practice?

## Type III Hypersensitivity — Immune Complex Disease **Key Point:** Type III hypersensitivity involves deposition of antigen–antibody complexes in tissues, activating complement and triggering inflammation. The **skin and joints** are the most commonly affected sites in clinical practice. ### Pathophysiology 1. **Antigen excess** (or antibody excess in some cases) → formation of small, soluble immune complexes 2. **Circulation and deposition** → complexes lodge in small blood vessels (especially at filtration barriers) 3. **Complement activation** → C3a, C5a generation → chemotaxis of neutrophils and macrophages 4. **Inflammation** → vasculitis, tissue damage ### Most Common Clinical Manifestations | Organ System | Manifestation | Frequency | |---|---|---| | **Skin** | Vasculitic rash, urticaria, petechiae | **Most common** | | **Joints** | Arthralgia, arthritis (non-erosive) | **Most common** | | Kidneys | Glomerulonephritis (membranoproliferative) | Common | | Lungs | Pulmonary vasculitis, hemorrhage | Less common | | Heart | Myocarditis, pericarditis | Rare | | Nervous system | Vasculitis, neuropathy | Rare | **High-Yield:** In **serum sickness** (the prototypical Type III reaction), the classic triad is **fever + rash + joint pain**, with skin and joints being the primary targets. ### Clinical Context of This Case - **Timeline**: 7 days after antibiotic exposure (typical for Type III: 3–10 days) - **Findings**: Maculopapular rash (skin), joint pain (joints), elevated ESR, immune complexes, low complement (C3, C4) → all hallmarks of Type III - **Diagnosis**: Drug-induced serum sickness ### Why Skin and Joints? **Clinical Pearl:** Skin and joints are affected most frequently because: - Small immune complexes preferentially deposit in **small blood vessels** of dermis and synovial membrane - High blood flow and filtration in these tissues promote complex deposition - Both are accessible for biopsy and clinical observation ### Mnemonic: "**SICK**" for Type III Manifestations - **S**kin (vasculitis, rash) - **I**mmune complexes (circulating) - **C**omplement (low C3, C4) - **K**idneys (glomerulonephritis, less common than skin/joints) ### Examples of Type III Diseases - **Serum sickness** (drug allergy, antiserum) - **Post-streptococcal glomerulonephritis** (kidneys predominate) - **Systemic lupus erythematosus** (multisystem, but skin and joints most visible) - **Rheumatoid arthritis** (joints predominate) - **Henoch–Schönlein purpura** (skin, joints, kidneys)