These are a focus of central fibrinoid necrosis surrounded by a palisade of epitheloid cells, occurring as a typical feature of sero-positive rheumatoid arthritis.
Subcutaneous nodules commonly occur on extensor surfaces subject to external pressure, for example, the upper forearm and elbow. Occasionally, they arise within the lungs or heart.
Nodules are rare in sero-negative rheumatoid arthritis.
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