A 42-year-old man with a 10-year history of rheumatoid arthritis presents with fever, joint pain, and a new palpable purpura over the lower extremities and buttocks. Laboratory studies show: WBC 7,200/μL, platelets 180,000/μL, ESR 68 mm/hr, CRP 12 mg/dL, serum creatinine 1.8 mg/dL (baseline 0.9), and urinalysis shows hematuria with RBC casts. Skin biopsy from the purpuric lesion shows leukocytoclastic vasculitis with IgA and IgG immune complex deposition on immunofluorescence. Which of the following best describes the pathogenic mechanism of this patient's skin and renal involvement?

Explanation

## Diagnosis: Type III Hypersensitivity (Immune Complex Vasculitis) **Key Point:** The combination of leukocytoclastic vasculitis, IgA/IgG immune complex deposition on immunofluorescence, glomerulonephritis with RBC casts, and systemic symptoms in a patient with chronic RA defines Type III hypersensitivity. ### Pathogenesis of Type III Hypersensitivity ```mermaid flowchart TD A[Chronic antigen exposure<br/>e.g., RA, infection, malignancy]:::outcome A --> B[Antibody production<br/>IgG, IgM, IgA]:::action B --> C[Antigen-antibody complex formation<br/>in slight antigen excess]:::action C --> D[Circulating immune complexes<br/>bypass clearance]:::action D --> E[Deposition in small vessels<br/>skin, glomeruli, joints]:::action E --> F[Complement activation<br/>C1q, C3, C5a]:::action F --> G[Neutrophil recruitment<br/>and degranulation]:::action G --> H[Leukocytoclasia<br/>vasculitis]:::outcome G --> I[Vessel wall damage<br/>RBC casts, hematuria]:::outcome ``` ### Mechanism of Immune Complex Deposition 1. **Complex formation:** Continuous antigen (e.g., RA-associated autoantigens) stimulates IgG and IgA antibody production. 2. **Optimal deposition:** Complexes formed in **slight antigen excess** (not antibody excess) are small enough to circulate but large enough to deposit in vessel walls. 3. **Complement activation:** IgG and IgA complexes activate the classical complement pathway (C1q binding), generating C3a and C5a. 4. **Neutrophil infiltration:** C5a recruits neutrophils, which phagocytose complexes and release proteolytic enzymes. 5. **Leukocytoclasia:** Nuclear debris from lysed neutrophils is visible on histology; vessel wall necrosis follows. **High-Yield:** The **3-week rule**: Type III reactions typically develop 7–21 days after antigen exposure (or continuously in chronic diseases like RA), unlike Type I (minutes) or Type IV (24–72 hours). ### Clinical Features of Type III Hypersensitivity | Feature | Details | |---------|----------| | **Skin** | Palpable purpura (small vessels), leukocytoclastic vasculitis on biopsy | | **Kidneys** | Glomerulonephritis, hematuria, RBC casts, proteinuria | | **Joints** | Arthritis, arthralgia (immune complex deposition in synovium) | | **Systemic** | Fever, malaise, elevated ESR/CRP | | **IF microscopy** | IgG, IgA, IgM, C3 deposition in vessel walls | **Clinical Pearl:** In RA, circulating immune complexes (CIC) are present in ~80% of patients; when they deposit systemically, they cause vasculitis, glomerulonephritis, and rheumatoid nodules. This is a hallmark of **rheumatoid vasculitis**, a serious extra-articular manifestation. ### Immunofluorescence Pattern - **Granular pattern** of IgG, IgA, C3 in vessel walls and glomeruli (vs. linear pattern in Type II, e.g., Goodpasture syndrome). - **Presence of C3** confirms complement activation—a key diagnostic clue for Type III. **Mnemonic: IMMUNE COMPLEX DISEASE** — **I**gG/IgA deposition, **M**ultiple organs (skin, kidney, joints), **M**edium/small vessels, **U**nder-clearance of complexes, **N**eutrophil infiltration, **E** **C**omplement activation, **C**irculating complexes, **O**ptimal antigen excess, **M**onths-to-years duration, **P**alpable purpura, **L**eukocytoclasia, **E**levated ESR, **X**-linked? (no—autosomal), **D**eposition in glomeruli, **I**mmunofluorescence granular, **S**ystemic symptoms, **E**xtra-articular RA. ### Differential: Type III vs. Other Hypersensitivities in Vasculitis | Type | Onset | Vessels | IF Pattern | Mediator | |------|-------|---------|------------|----------| | **Type I** | Minutes | Mast cells, not vasculitis | N/A | Histamine | | **Type II** | Hours–days | Medium/large | **Linear** IgG, C3 | Antibody-mediated | | **Type III** | 7–21 days | Small | **Granular** IgG, IgA, C3 | Immune complexes | | **Type IV** | 24–72 hrs | Granulomatous | No IF | T cell infiltration | [cite:Robbins 10e Ch 6]