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 an allergic reaction to inhaled organic dusts or, less commonly, chemicals.
Many types of dust can cause allergic 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. Air conditioner lung is another example. It occurs when contaminated humidifiers or air conditioners (especially large systems in office buildings) circulate antigens that are capable of causing a hypersensitivity reaction.
Only a small number of people who inhale these common dusts develop allergic reactions. Only a small percentage of those people who develop allergic reactions suffer irreversible damage to the lungs. 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. 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 and Diagnosis
If a person has developed hypersensitivity to an organic dust, then fever, cough, chills, and shortness of breath typically appear 4 to 8 hours after re-exposure to it. Wheezing is unusual. If the person has no further contact with the antigen, symptoms usually diminish over a day or two, but complete recovery may take weeks.
In a slower form of hypersensitivity pneumonitis (subacute form), cough and shortness of breath may develop over days or weeks and sometimes may be so severe that the person needs to be hospitalized.
With chronic hypersensitivity pneumonitis, a person repeatedly comes in contact with an antigen over months to years, and lung scarring (fibrosis) may result. Shortness of breath during exercise, cough, fatigue, and weight loss may gradually progress over months or years. Eventually, the disease may lead to respiratory failure (see Respiratory Failure and Acute Respiratory Distress Syndrome: 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 depends on the clinical features, identification (if possible) of the dust or other substance causing the problem, and evidence of the person's exposure to the suspected agent, as determined by the presence of antibodies on a blood test.
Doctors may suspect the diagnosis based on finding something abnormal on a chest x-ray. Results of pulmonary function tests (see Diagnosis of Lung Disorders: 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. When the antigen cannot be identified and the diagnosis is in doubt, re-exposing the recovered person to the antigen that is thought to be responsible and observing the person for symptoms or changes in lung function may occasionally be useful to confirm the diagnosis.
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). They may remove the tissue while examining the airways using a viewing tube (bronchoscopy—see Diagnosis of Lung Disorders: Bronchoscopy). Sometimes, rather than (or in addition to) removing tissue by using a sharp instrument, the person performing bronchoscopy may wash out the lung with fluid (bronchoalveolar lavage) to extract cells for examination. Rarely, a different type of viewing tube inserted through the chest wall (thoracoscopy—see Diagnosis of Lung Disorders: Thoracoscopy) may be used to examine the lung surface and pleural space, or an operation in which the chest wall is opened (thoracotomy) may be needed to obtain larger pieces of lung tissue (see Diagnosis of Lung Disorders: Thoracotomy).
Prevention and Treatment
The best prevention is to avoid exposure to the antigen, but this may be impractical 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 September 2006 by Lee S. Newman, MD, MA