Extrinsic allergic alveolitis (hypersensitivity pneumonitis or farmer's lung)

Extrinsic allergic alveolitis (or hypersensitivity pneumonitis) is a rare interstitial lung disease caused by inhaling antigens in dust, gases, or aerosols. The pathogenesis of extrinsic allergic alveolitis involves both type III and type IV hypersensitivity reactions that are mediated by immune complexes and TH1 T cells, respectively. Knowledge of the diverse clinical disease forms and the different courses of disease is important for discovering potential antigens. An early diagnosis with the possibility of allergen avoidance should always be pursued. Persistent antigen exposure can cause lung fibrosis and emphysema, leading to respiratory failure. This review focuses on the epidemiology, etiology, immunopatho-genesis, pathology, and clinical features of extrinsic allergic alveolitis. In addition, diagnostic tools such as determination of precipitins, bronchoalveolar lavage, and radiologic techniques are discussed. An overview of current treatment options and the prognoses of different disease courses is given at the end.