What type of infiltrate would be expected with immune-mediated arthritis?

• A. Neutrophilic infiltrate
• B. Monocytic infiltrate
• C. Eosinophilic infiltrate
• D. Lymphocytic infiltrate

Answer: (D)

The key idea is that immune-mediated arthritis is driven by the adaptive immune response, so the joint inflammation is primarily mononuclear. In the synovium, you typically see lymphocytes, often with plasma cells, reflecting T and B cell–mediated activity against joint components. This lymphocytic infiltrate signals a chronic, antigen-driven process. Neutrophils would point to infectious or acute inflammatory processes, eosinophils to allergic or parasitic reactions, and macrophage-dominated infiltrates to certain granulomatous or non‑classic chronic conditions. So the histologic pattern most characteristic of immune-mediated arthritis is a lymphocytic infiltrate.