Definition 
Hypersensitivity pneumonitis is a pulmonary disease characterized by an immune (allergic) inflammation of interstitium and alveolar wall.

Pathology
Interstitial pneumonitis (mononuclear inflammatory cells), fibrosis and occasionally, granulomas. Refer to Figure 15-37 in your textbook.

Etiology/Pathogenesis
The etiology is varied (from spores to cotton fibers). The pathogenesis is a hypersensitivity (allergic) reaction to inhaled, foreign antigens.

Pathophysiology
Fever, respiratory difficulty several hours after exposure to the antigen. Chronic form can lead to pulmonary insufficiency.