Immunology of hypersensitivity pneumonitis

Hypersensitivity pneumonitis (extrinsic allergic alveolitis) represents a spectrum of granulomatous, interstitial, and alveolar-filling lung disorders of which farmer's lung is a classic example. A major source of offending antigens in these diseases are thermophilic actinomycetes growing in moldy vegetable matter especially Micropolyspora faeni, and members of the Thermoactinomyces genus. Acutely, hypersensitivity pneumonitis presents as cough, dyspnea and fever, with crepitant rales, leucocytosis, diffuse interstitial and alveolar pulmonary infiltrates and a restrictive-type pulmonary functional deficit. Symptoms usually begin 4 to 6 hr after exposure to large quantities of causative organic dust. Chronically, these diseases may present with the gradual onset of cough, dyspnea on exertion, fatigue, anorexia, and weight loss which may progress to pulmonary fibrosis or severe pulmonary insufficiency. While early ideas on the pathogenesis of hypersensitivity pneumonitis support the role of Type III immune complex hypersensitivity, more recent evidence attests to the important and integral role of Type IV or delayed-type hypersensitivity. It is the purpose of this review, therefore, to describe those immune mechanisms relevant to the pathogenesis of hypersensitivity pneumonitis and stress the importance of "local" pulmonary immune responsiveness.