Interstitial pneumonitis (mononuclear inflammatory cells), fibrosis and occasionally, granulomas. Refer to Figure 15-37 in your textbook.
The etiology is varied (from spores to cotton fibers). The pathogenesis is a hypersensitivity (allergic) reaction to inhaled, foreign antigens.
Fever, respiratory difficulty several hours after exposure to the antigen. Chronic form can lead to pulmonary insufficiency.