Immunohistochemical classification of cutaneous pseudolymphomas: delineation of distinct patterns.

Because of the broad spectrum of clinical and histological features, cutaneous pseudolymphomas are difficult to classify. To delineate objective criteria for classification, we investigated the immunoarchitecture of 53 cases of pseudolymphomas; 29 were classified as T cell pseudolymphomas. The immunohistologic characteristics were the absence of B cell compartments, the predominance of T helper-inducer cells and the presence of Langerhans cells/indeterminate cells. Lymphomatoid contact dermatitis showed the bandlike (superficial) T cell pattern. Lymphocytic infiltration of the skin, lymphomatoid papulosis, lymphomatoid drug reactions, and persistent nodules following assaults by arthropods revealed a nodular T cell pattern. Twenty-four cases represented B cell pseudolymphomas containing a nodular arrangement of B lymphocytes. In 6 lesions, there were B cell aggregates without the association of dendritic reticulum cells (non follicular B cell pattern); in 18, the B cell clusters were associated with dendritic reticulum cells and a typical expression of IgM and IgD, thus forming fully developed germinal centers (follicular B cell pattern). The B cell clusters were always surrounded by distinct T zones. B cell patterns were present in lymphadenosis benigna cutis, large cell lymphocytoma and occasionally, in persistent nodules, following assaults by arthropods.