- Hypersensitivity pneumonitis is also known as extrinsic allergic alveolitis
- 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.
- Most exposures are occupational, but hobbies such as bird breeding are also implicated.
- Most common and well-studied forms of hypersensitivity pneumonitis are farmer's lung and bird fancier's lung.
- Treatment is:
- Cessation of exposure to causative agent
 Imaging Findings
Acute hypersensitivity pneumonitis
- Chest radiographs in affected patients may be normal; thin-section CT can be helpful in these patients for showing characteristic centrilobular ground glass or nodular opacities.
- When abnormal, chest radiographs typically show focal or diffuse heterogeneous or homogeneous opacities.
Subacute hypersensitivity pneumonitis
- Patients with subacute disease usually have a more indolent clinical presentation. Nodular opacities are a characteristic feature on chest radiographs and CT. Centrilobular ground-glass or nodular opacities on CT suggest the diagnosis.
- Head cheese sign
Chronic hypersensitivity pneumonitis
- Chronic disease typically manifests with upper lung zone fibrosis. Characteristic distribution and presence of centrilobular nodules on CT help distinguish chronic hypersensitivity pneumonitis from idiopathic pulmonary fibrosis.
Patient #1: Patient who fancies birds presents with SOB
 See Also
 External Links
- Matar, Luke D., McAdams, H. Page, Sporn, Thomas A. Hypersensitivity Pneumonitis. Am. J. Roentgenol. 2000 174: 1061-1066.