Pulmonary lymphoid disorders.

Organized lymphoid aggregates are normally found within the walls of bronchi in many species and may occur, under conditions of disease, in humans. This bronchus-associated lymphoid tissue (BALT) can be viewed as an organizing principle to explain the behavior and distribution of many pulmonary lymphoid proliferations, both hyperplastic and neoplastic. The extent of lymphoid hyperplasia can vary, from multifocal proliferations that arise in and remain in the airway walls (follicular hyperplasia of BALT) to those that form a solitary mass or nodule (nodular lymphoid hyperplasia of BALT or "pseudolymphoma") to multifocal or diffuse lymphoid hyperplasia of BALT ("lymphoid interstitial pneumonitis"). It seems likely that more than one cause accounts for these proliferations. Of interest is the hypothesis that many examples of diffuse lymphoid hyperplasia associated with the acquired immune deficiency syndrome may have a viral cause, possibly human immunodeficiency virus or, in some cases, Epstein-Barr virus (EBV). Most pulmonary lymphomas are low-grade B cell lymphomas. They exhibit histological diversity in any given case, characterized by small lymphocytes with irregular nuclei and pale cytoplasm (so-called "centrocytelike" cells), scattered immunoblasts, and lymphoplasmacytic or plasma cells. Reactive germinal centers are frequently present, and this, along with the benign clinical behavior of these tumors, may cause diagnostic confusion with reactive lesions. Both the histological appearance and the clinical behavior (in particular, the tendency to recur in extranodal sites) of these low-grade lymphomas can be explained on the basis of origin in BALT. T cell lymphoproliferative processes can occur in the lung but are rare. Lymphomatoid granulomatosis (angioimmunoproliferative lesion) is an angiocentric and necrotizing, polymorphous lymphoid lesion that presents as multiple masses in the lung, involves skin and central nervous system (among other organs), may progress to histologically overt lymphoma, and is immunophenotypically predominantly a T cell process. Microscopically, a lymphohistiocytic infiltrate, including variable numbers of atypical cells, is present. Recent in situ hybridization and immunohistochemical studies have served to show EBV localized to the large cells. In addition, these atypical cells often show B cell immunophenotypic features and are present in a background of small, reactive T cells. It is therefore possible that lymphomatoid granulomatosis is a family of T cell rich lymphoproliferations driven by EBV infection of B cells and/or T cells, analagous to the EBV-associated, posttransplantation B cell lymphoproliferative disorders.