Hypersensitivity pneumonitis

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Hypersensitivity pneumonitis is a type of inflammation in and around the tiny air sacs (alveoli) and smallest airways (bronchioles) of the lung caused by a hypersensitivity reaction to inhaled organic dusts or, less commonly, chemicals.

Dusts that contain microorganisms or proteins may cause a hypersensitivity reaction in the lungs.
People may develop fever, cough, chills, and shortness of breath within 4 to 8 hours of re-exposure to substances to which they are sensitized.
Doctors use chest x-rays and tests of lung function to determine whether there is a problem with the lungs.
The substance that is causing the reaction can sometimes be identified by using a blood test and, when the person is affected at work, an industrial hygiene specialist may analyze the workplace to identify triggering substances.
People who work with substances that are likely to cause hypersensitivity reactions should use protective equipment, such as face masks, during work.
People who can avoid re-exposure usually recover, but they sometimes need to take corticosteroids to reduce lung inflammation.

(See also Overview of Interstitial Lung Diseases.)
In these hypersensitivity reactions (also called allergic reactions), the immune system attacks something in an organic dust or chemical the person inhales. Substances released by cells of the immune system damage the lungs, where the dust has lodged. The part of the inhaled dust that triggers the immune reaction is called an antigen.