Hypersensitivity pneumonitis: correlation of cellular and immunologic changes with clinical phases of disease.

Many airborne organic antigens in the form of fungal spores and thermophilic bacteria or dander and aerosolized proteins can cause hypersensitivity pneumonitis after repeated inhalational exposure. Although a syndrome of acute illness often occurs, many people who are exposed and become sensitized can remain asymptomatic. However, immunologic evidence from an analysis of cells and antibodies obtained by bronchoalveolar lavage from asymptomatic subjects may indicate an ongoing alveolitis. With chronic illness, which has many characteristics of an interstitial fibrotic pneumonitis, a lymphocytic alveolitis and granulomatous reaction develop in the lungs. This review emphasizes the immunological changes in the lungs that occur at different phases of hypersensitivity pneumonitis and contribute to current concepts about pathogenetic mechanisms.