Hypersensitivity pneumonitis (extrinsic allergic alveolitis) 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.
In these hypersensitivity 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. Hypersensitivity reactions are different from typical allergic reactions (see Overview of Allergic Reactions).
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 feathers of birds (either on living birds or in pillows and comforters) is inhaled.
Lung damage appears to result from damage done by lymphocytes, a type of white blood cell. 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.
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Depending on how quickly symptoms develop, hypersensitivity pneumonitis may be
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 (see Respiratory Failure). 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.
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, the presence of antibodies on a blood test, or a combination.
Doctors may suspect the diagnosis based on chest x-ray findings. Usually, however, computed tomography (CT) is needed to help confirm the diagnosis. Results of pulmonary function tests (see 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–—see Thoracoscopy) and also used to examine the lung surface and pleural space, or an operation in which the chest wall is opened (thoracotomy) may be needed (see Thoracotomy). 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.
Prevention and Treatment
The best prevention is to avoid exposure to the antigen, but this 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.
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.
Last full review/revision October 2013 by Harold R. Collard, MD