Hypersensitivity pneumonitis.

Hypersensitivity pneumonitis represents a disease spectrum and not a single pulmonary disorder. Both acute and chronic forms of disease occur and presumably reflect different yet interrelated immune mechanisms of pathogenesis. The diagnosis of hypersensitivity pneumonitis, although often straightforward, may depend on the use of immunologic and microbiologic analyses with the aid of inhalation challenge testing. Finally, the immune pathway to disease expression is as yet undefined and may represent subtle changes in "suppressor" cell populations and their regulatory effects on immune effector cells.