Rosai-Dorfman disease, also called sinus histiocytosis with massive lymphadenopathy, is most commonly seen in cervical lymph nodes but can involve extranodal sites in about 25% of cases. In some cases, extranodal manifestations are the sole areas of involvement. As seen here at low power, the disease is a histiocytic process and can resemble hematolymphoid disorders. Small bowel mucosa can be seen at the upper left. The submucosa contains a diffuse infiltrate composed of histiocytes, lymphocytes, and plasma cells.