The paracortical area in dermatopathic lymphadenitis and other reactive conditions of the lymph node.

We compared the paracortical area in 4 cases of dermatopathic lymphadenitis (DL) with the same area in 11 cases of various other reactive conditions of the lymph node by immuno- and enzymehistochemical techniques. In addition, electron microscopy was performed on three cases of DL. The paracortical nodules in DL proved to be composed of a variable number of dendritic, OKT6+ OKIa + ATPase+ cells, admixed with helper T-lymphocytes. All other lymph nodes studied lacked dendritic OKT6+ cells, whereas OKIa positivity was found in the cortical (follicular centers and mantle zones) and paracortical area (lymphocytes and scattered dendritic cells). Short incubation for ATPase revealed a paracortical, pericellular staining pattern in cases of DL, whereas in all other cases this staining pattern was observed only after long incubation times. On electron microscopy, three types of dendritic cells were found in DL, namely interdigitating reticulum cells ( IDRC ). Langerhans cells (LC) and macrophages. Intermediate forms between IDRC and LC, containing a few Birbeck granules and a well developed rough endoplasmic reticulum, were found. It is suggested that immunoreactivity for the monoclonal antibody OKT6 is restricted to cases of DL, and is due to the appearance of dendritic cells that have LC-characteristics. These cells either arrive from the skin along afferent lymph vessels, or are the result of a local transformation process of IDRC that acquire LC-characteristics, i.e. OKT6 immunoreactivity and Birbeck granules.