## Histopathology of Rheumatoid Arthritis **Key Point:** The hallmark histological lesion of RA is the **rheumatoid nodule**, which consists of fibrinoid necrosis surrounded by a palisade of epithelioid histiocytes and outer rim of chronic inflammatory cells (lymphocytes, plasma cells, fibroblasts). ### Rheumatoid Nodule Structure 1. **Central fibrinoid necrosis** — area of tissue death with fibrin deposition 2. **Palisade layer** — epithelioid histiocytes and fibroblasts arranged in a fence-like pattern around the necrotic center 3. **Outer rim** — chronic inflammatory infiltrate (lymphocytes, plasma cells) and fibroblasts ### Synovial Changes in RA **High-Yield:** Early RA synovium shows: - Villous hypertrophy and hyperplasia - Inflammatory cell infiltration (T cells, B cells, macrophages) - Synovial lining hyperplasia - Pannus formation (granulation tissue with inflammatory cells) **Clinical Pearl:** Rheumatoid nodules occur in 20–30% of seropositive (RF+) RA patients and are rare in seronegative RA. They typically appear on pressure points (elbows, fingers) and indicate more severe, erosive disease. ### Comparison of Inflammatory Lesions | Feature | RA Nodule | Caseating Granuloma (TB) | Acute Suppuration | |---------|-----------|--------------------------|-------------------| | Central necrosis | Fibrinoid (non-caseating) | Caseating | Pus with neutrophils | | Surrounding cells | Epithelioid palisade | Epithelioid palisade | Neutrophils only | | Outer rim | Chronic inflammation | Fibrosis, giant cells | Minimal | | Acid-fast bacilli | Absent | Present (Ziehl-Neelsen+) | Absent | **Mnemonic:** **PELF** — Palisade, Epithelioid, Lymphocytes, Fibrinoid necrosis (the components of an RA nodule). [cite:Robbins 10e Ch 26]
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