Airway involvement in hypersensitivity pneumonitis.

Exposure to organic particles causes, in a susceptible host, diffuse inflammation of the lung acinus. However, immunopathologic response may not be confined to the alveoli and may also involve the large and peripheral airways. Therefore, after allergen inhalation a clinical spectrum of respiratory disorders may be observed, including hypersensitivity pneumonitis, asthma, chronic airway obstruction, and simple chronic bronchitis. Hypersensitivity pneumonitis is not a uniform disease but a complex syndrome, and the involvement of the airways may occur alone, simultaneously with, or after the parenchymal disease.