Hypersensitivity pneumonitis is an inflammation of the lungs that occurs from exposure to certain allergens. It is a complex syndrome rather than a single disease, and symptoms can vary in appearance and intensity. Hypersensitivity pneumonitis usually affects the tiny air sacs of the lungs (alveoli).

People develop hypersensitivity pneumonitis after being sensitized during repeated exposures to an allergen. Organic dust, especially from bird feces, is the most common source of sensitization and symptoms. Other sources of this dust include animal proteins (including dander, urine and feces), molds, plants, hay and foods.

There are two forms of hypersensitivity pneumonitis:

• Acute hypersensitivity pneumonitis. Usually occurs within four to 12 hours after exposure to an allergen. Symptoms may persist for up to 18 hours or longer, but usually begin to resolve soon after exposure to the allergen ceases.
  
  
• Chronic hypersensitivity pneumonitis. A result of several bouts of the acute form of the disease, it can lead to a scarring of the lung tissue (pulmonary fibrosis) that often is not reversible.

Early diagnosis and treatment of hypersensitivity pneumonitis is essential. The prognosis is excellent if treatment begins when the disease is in its acute phase. However, once the disease becomes chronic, scarring of the lungs (fibrosis) can set in. This is often irreversible and can be debilitating for the patient.