Hypersensitivity pneumonitis

Last updated: October 19, 2021

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Hypersensitivity pneumonitis (or extrinsic allergic alveolitis) is a hypersensitivity reaction following exposure to environmental allergens. It is associated with inflammatory interstitial lung disease. Occupational groups affected by hypersensitivity pneumonitis are most commonly exposed to birds, hay, or certain reactive chemical species. Hypersensitivity pneumonitis can be classified into acute, subacute, or chronic forms depending on the frequency, length, and intensity of exposure, severity of symptoms, and duration of subsequent illness. The acute form is characterized by fever, flu-like symptoms, and leukocytosis. The subacute form is characterized by the insidious onset of productive cough, dyspnea, and fatigue over weeks to months. Chronic hypersensitivity pneumonitis is characterized by long-term progressive dyspnea, weight loss, cough, and fatigue. Diagnosis is usually based on occupational history and evidence of fibrosis on imaging, bronchoalveolar lavage with lymphocytosis, positive inhalation challenge testing, or histopathology showing poorly formed noncaseating granulomas or mononuclear cell infiltrate. The preferred treatment for the acute form includes allergen avoidance (e.g., change of occupation) and administration of glucocorticoids for symptomatic relief. The prognosis for the acute form is good. Once severe damage to lung structures has occurred, however, few treatment options remain. Chronic hypersensitivity pneumonitis may lead to pulmonary fibrosis, which may ultimately require transplantation.

Overview of antigens and diseases
Antigen Source [2] Disease
Animal proteins
  • Avian proteins
  • Exposure to droppings, serum, or feathers of birds (e.g., parakeets, canaries, chickens, pigeons)
  • Pigeon breeder's lung
  • Actinomycete spores from air conditioners, humidifiers, and water reservoirs
  • Humidifier lung (air-conditioner setting)
  • Actinomycete spores from moldy hay
  • Farmer's lung
  • Actinomycete spores from sugar cane
  • Bagassosis
  • Penicillium casei or P. roqueforti spores from cheese casings
  • Cheese washer's lung
  • Actinomycete spores from moldy compost
  • Mushroom worker's lung (compost treatment)
  • Malt worker's lung
  • Grain weevil dust
  • Grain handler's lung
  • Various bacteria in sawdust (through logging)
  • Woodworker's lung
  • Hot tub lung [3]
Chemical substances
  • Chemical worker's lung

Smokers are less likely to be symptomatic because they have a decreased immune response to new antigens.

  • Acute (commencing 4–8 hours after exposure) [2]
  • Subacute (weeks to months after continuous exposure)
    • Insidious onset of fatigue
    • Possible progression to persistent productive cough and dyspnea
    • May begin noticing weight loss
  • Chronic (months after continuous exposure)

A recurrent common cold with an irritating cough and fever may indicate hypersensitivity pneumonitis.

Primarily a clinical diagnosis based on history of exposure and typical clinical presentation, which is supported by the presence of any one of the following:

In hypersensitivity pneumonitis, IgG antibodies are found during an allergic reaction.

The differential diagnoses listed here are not exhaustive.

We list the most important complications. The selection is not exhaustive.

  • Favorable in the acute stage, but the disease recurs and worsens upon re-exposure
  • Worsens with severity of fibrosis
  1. Reportable Diseases and Conditions. https://www.health.ny.gov/professionals/reportable_conditions/reportable_conditions.htm. Updated: January 1, 2011. Accessed: May 5, 2017.
  2. Hypersensitivity Pneumonitis (Extrinsic Allergic Alveolitis). http://www.merckmanuals.com/professional/pulmonary-disorders/interstitial-lung-diseases/hypersensitivity-pneumonitis. Updated: April 1, 2016. Accessed: May 5, 2017.
  3. Yasin H, Mangano WE, Malhotra P, Farooq A, Mohamed H. Hot Tub Lung: A Diagnostic Challenge.. Cureus. 2017; 9 (8): p.e1617. doi: 10.7759/cureus.1617 . | Open in Read by QxMD
  4. D'souza RS, Donato A. Hypersensitivity pneumonitis: an overlooked cause of cough and dyspnea.. Journal of community hospital internal medicine perspectives. 2017; 7 (2): p.95-99. doi: 10.1080/20009666.2017.1320202 . | Open in Read by QxMD
  5. Wollin L, Distler JHW, Redente EF, et al. Potential of nintedanib in treatment of progressive fibrosing interstitial lung diseases.. The European respiratory journal. 2019; 54 (3). doi: 10.1183/13993003.00161-2019 . | Open in Read by QxMD
  6. Shibata S, Furusawa H, Inase N. Pirfenidone in chronic hypersensitivity pneumonitis: a real-life experience.. Sarcoidosis, vasculitis, and diffuse lung diseases : official journal of WASOG. 2018; 35 (2): p.139-142. doi: 10.36141/svdld.v35i2.6170 . | Open in Read by QxMD
  7. Herold G. Internal Medicine. Herold G ; 2014

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