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In Rosai-Dorfman disease (RDD), the affected lymph nodes are enlarged with thickened fibrotic capsules. The normal nodal architecture is effaced by massively expanded sinuses containing many histiocytes, small lymphocytes, plasma cells and a few eosinophils and neutrophils. The histiocytes have abundant clear or pale-staining foamy cytoplasm and a large, round or oval, vesicular nucleus with a single prominent nucleolus.
The histiocytes show non-destructive phagocytosis of lymphocytes and plasma cells within their cytoplasm - a phenomenon known as emperipolesis (described in more detail later). In addition to sinusoidal dilatation, there may be reactive lymphoid follicles in the cortex. The medullary areas of the node contain numerous plasma cells. Features that are absent include increased mitotic activity, necrosis and granulomas.
The histiocytes show non-destructive phagocytosis of lymphocytes and plasma cells within their cytoplasm - a phenomenon known as emperipolesis (described in more detail later). In addition to sinusoidal dilatation, there may be reactive lymphoid follicles in the cortex. The medullary areas of the node contain numerous plasma cells. Features that are absent include increased mitotic activity, necrosis and granulomas.