MSD Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

Hypersensitivity Pneumonitis

(Extrinsic Allergic Alveolitis)


Joyce Lee

, MD, MAS, University of Colorado School of Medicine

Reviewed/Revised Jul 2023
Topic Resources

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 computed tomography (CT) 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.

Causes of Hypersensitivity Pneumonitis

Many substances can cause hypersensitivity reactions in the lungs. Organic dusts that contain microorganisms or proteins, and chemicals, such as isocyanates, may cause hypersensitivity pneumonitis. Farmer's lung, which results from repeated inhalation of heat-loving (thermophilic) bacteria in moldy hay, is a well-known example of hypersensitivity pneumonitis. Bird fancier's lung is another example. It occurs when dust from the droppings or feathers of birds (either on living birds or in pillows and comforters) is inhaled.

Did You Know...

  • Only a small number of people who inhale certain common dusts develop hypersensitivity reactions. Generally, a person must be exposed repeatedly over time before sensitivity and resultant disease develop.

Lung damage appears to result from damage done by lymphocytes, a type of white blood cell White blood cells The immune system is designed to defend the body against foreign or dangerous invaders. Such invaders include Microorganisms (commonly called germs, such as bacteria, viruses, and fungi) Parasites... read more . Initial exposures to the dusts sensitize lymphocytes. Some lymphocytes then help to produce antibodies that play a role in tissue damage. Other lymphocytes participate directly in inflammation after subsequent antigen exposure. Recurrent exposure to the antigen results in a chronic inflammatory response, which is manifested by a buildup of white blood cells in the walls of the alveoli and small airways. This buildup leads progressively to symptoms and disease.


Symptoms of Hypersensitivity Pneumonitis

Depending on how quickly symptoms develop, hypersensitivity pneumonitis may be

  • Acute

  • Subacute

  • Chronic

In acute hypersensitivity pneumonitis, people develop fever, cough, chills, and shortness of breath typically 4 to 8 hours after re-exposure to significant amounts of the causative organic dust. Wheezing is unusual. If people have no further contact with the antigen, symptoms usually diminish over a day or two, but complete recovery may take weeks.

Subacute hypersensitivity pneumonitis develops more slowly. Cough and shortness of breath may develop and worsen over days or weeks. Sometimes symptoms may be so severe that people need to be hospitalized.

With chronic hypersensitivity pneumonitis, people repeatedly come in contact with an antigen over months to years, and lung scarring (fibrosis) may result. Shortness of breath during exercise, cough, and fatigue, may gradually progress over months or years. Eventually, the disease may lead to respiratory failure Respiratory Failure Respiratory failure is a condition in which the level of oxygen in the blood becomes dangerously low or the level of carbon dioxide in the blood becomes dangerously high. Conditions that block... read more . Older people may be more prone to chronic, progressively worsening disease because they have been exposed to an antigen for a long period of time.

Diagnosis of Hypersensitivity Pneumonitis

  • Chest computed tomography

The diagnosis of hypersensitivity pneumonitis is based partly on symptoms, the clinical features, identification (if possible) of the dust or other substance causing the problem, as determined by what the person says, an analysis of the workplace by industrial hygiene specialists, , or a combination.

Doctors may suspect the diagnosis based on chest x-ray findings. Usually, however, computed tomography (CT) of the chest is needed to help confirm the diagnosis. Results of pulmonary function tests Pulmonary Function Testing (PFT) Pulmonary function tests measure the lungs' capacity to hold air, to move air in and out, and to absorb oxygen. Pulmonary function tests are better at detecting the general type and severity... read more Pulmonary Function Testing (PFT) —which measure the lungs' capacity to hold air and their ability to move air in and out and to exchange oxygen and carbon dioxide—are used to assess how well the lungs work and may help support a diagnosis of hypersensitivity pneumonitis.

In cases that are not clear, especially when an infection is suspected, doctors may remove small pieces of lung tissue for examination under a microscope (lung biopsies). To obtain a tissue sample, a viewing tube may need to be inserted through the chest wall (thoracoscopy Thoracoscopy Thoracoscopy is the visual examination of the lung surfaces and pleural space through a viewing tube (a thoracoscope). Thoracoscopy is used to view the lung and the space surrounding the lungs... read more ) and also used to examine the lung surface and pleural space, or an operation in which the chest wall is opened (thoracotomy Thoracotomy Thoracotomy is an operation in which the chest wall is opened to view the internal chest organs, to obtain samples of tissue for laboratory examination, and to treat disorders of the lungs,... read more ) may be needed. Sometimes, rather than (or in addition to) removing tissue by using a sharp instrument, the doctor doing the bronchoscopy may wash out the lung with fluid (bronchoalveolar lavage) to extract cells for examination.

Occasionally, blood tests are needed to look for clues to the substance causing the hypersensitivity or to eliminate other possible causes.

Treatment of Hypersensitivity Pneumonitis

  • Corticosteroids or immunosuppressants

People who have an acute episode of hypersensitivity pneumonitis usually recover if further contact with the substance is avoided. If the episode is severe, corticosteroids, such as prednisone, reduce symptoms and may help reduce severe inflammation. Prolonged or recurring episodes may lead to irreversible disease and progressive disability and require long-term immunosuppression.

Prevention of Hypersensitivity Pneumonitis

The best prevention is to avoid exposure to the antigen, but avoiding exposure may be impractical (for example, if the person cannot change jobs). Eliminating or reducing dust, wearing protective masks, and using good ventilation systems may help prevent both sensitization and recurrence. However, even the best prevention methods may not be effective.

quiz link

Test your knowledge

Take a Quiz!