Luke D. Matar ; H. Page McAdams ; Thomas A. Sporn
Hypersensitivity pneumonitis, also known as extrinsic allergic alveolitis, is an inflammatory lung disease caused by inhalation of airborne organic particulate matter. These particles, which are usually 1-5 µm in diameter, deposit in distal air spaces and produce an immune-mediated inflammatory response in sensitized individuals. Causative agents are numerous and include bacteria, fungi, avian proteins, and wood dusts  (Table 1). Most exposures are occupational, but hobbies such as bird breeding are also implicated. The most common and well-studied forms of hypersensitivity pneumonitis are farmer’s lung and bird fancier’s lung .