Immunohistochemistry in Lymph Node Histiocytosis

Reactive sinus histiocytosis of lymph nodes is a very common finding , and undoubtedly in many cases it is largely ignored, since it is often of little or no clinical importance. However, when this finding becomes prominent, it can result in a somewhat alarming appearance. The extreme end of this spectrum occurs in cases of reactive histiocytosis related to prior joint replacement surgery, which can closely simulate metastatic carcinoma in some cases. As its name implies, the proliferating histiocytes occur within the lymph node sinuses, and they characteristically express CD68 , but lack S100 protein and CD1a. One typically finds a minority of histiocytes in the background that express S100 protein and CD1a, but the large majority of histiocytes in reactive histiocytosis do not express these markers.