## Pathological Hallmark of Rheumatoid Arthritis **Key Point:** Pannus formation is the cardinal pathological feature of RA. Pannus is a mass of inflammatory granulation tissue composed of fibroblasts, macrophages, and lymphocytes that invades and destroys articular cartilage and subchondral bone. ### Pannus Formation Process 1. Chronic synovial inflammation with T-cell and B-cell infiltration 2. Proliferation of synovial lining cells (hyperplasia) 3. Formation of granulation tissue (pannus) at the cartilage-pannus junction 4. Enzymatic degradation of cartilage matrix by collagenase and matrix metalloproteinases (MMPs) 5. Erosion of subchondral bone via osteoclast activation ### Microscopic Features of Pannus - Proliferating fibroblasts and myofibroblasts - Dense infiltration of lymphocytes (CD4+ T cells predominant), plasma cells, and macrophages - Neovascularization - Fibrin deposition - Absence of normal synovial architecture **High-Yield:** The pannus-cartilage junction is where active destruction occurs. This is why early aggressive DMARD therapy is critical — once pannus invades and erodes cartilage and bone, structural damage becomes irreversible. **Clinical Pearl:** Pannus can be visualized on MRI and ultrasound as a hypoechoic or intermediate-signal tissue mass at the joint margin, and its presence correlates with radiographic progression.
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