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

Pathology

• The pathologic features are the same regardless of etiology
• Triad of findings: chronic interstitial inflammation (mononuclear inflammatory cells), chronic bronchiolitis and small non-necrotizing granulomas. Between involved areas, there is usually normal lung parenchyma

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.