The lung may be the site of organ-specific and non-organ-specific immunopathologic changes. These alterations are operational at two defensive locations, the bronchovascular "barrier" and the alveolovascular "barrier." The subepithelial bronchovascular barrier concentrates plasma proteins and fibrogen in asthma and similar disorders. The alveolovascular barrier is the site of localization of autoantibody directed against alveolar basement membrane in patients with Goodpasture's syndrome and in experimental pneumotoxic alveolitis. Immunopathogenic mechanisms may operate in a variety of pulmonary diseases. Further investigation is encouraged.