Hypersensitivity pneumonitis

Last updated: March 31, 2023

Summarytoggle arrow icon

Hypersensitivity pneumonitis (HP) is a hypersensitivity reaction caused by inhalation of environmental and/or occupational antigens, manifesting in interstitial lung disease (ILD). Individuals affected by HP are most commonly exposed to animal proteins (especially birds), fungi, bacteria, or inorganic chemicals. HP can be classified as fibrotic or nonfibrotic based on findings on high-resolution CT (HRCT) of the chest. Onset may be acute with constitutional symptoms, flu-like symptoms, and leukocytosis, or more insidious (e.g., cough, dyspnea, fatigue, and weight loss). Diagnosis is based on exposure history and evidence of fibrosis on imaging, lymphocytosis on bronchoalveolar lavage, and histopathology showing poorly formed noncaseating granulomas or mononuclear cell infiltrate. Most acute episodes are self-limited and resolve with antigen avoidance. Patients with persistent symptoms after antigen removal often receive immunosuppressants (e.g., glucocorticoids). Patients with refractory fibrotic HP may ultimately require lung transplantation.

Etiologytoggle arrow icon

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.

Clinical featurestoggle arrow icon

Patients with chronic HP do not necessarily have a history of acute HP. [7]

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

Diagnosticstoggle arrow icon

The diagnosis of HP is based on a history of antigen exposure, typical clinical and radiologic features, and supportive diagnostic studies.

Approach [7][8]

Clinical improvement after antigen avoidance supports the diagnosis of HP, but lack of improvement should not be used to rule out the diagnosis. [7]

High-resolution CT [7][8]

Findings are used to classify HP as fibrotic or nonfibrotic (i.e., purely inflammatory), which impacts further workup, treatment, and prognosis.

Atypical HRCT findings for HP (e.g., usual interstitial pneumonia pattern) may still be compatible with HP. Specialist consultation is recommended for determining the likelihood of HP versus other causes of ILD. [7]

Classification of HP patients into acute, subacute, or chronic HP does not have clear treatment or prognostic implications and is no longer recommended; patients are now classified based on the presence or absence of fibrosis. [7][8]

Additional studies [7][8]

Additional studies should be considered in consultation with a multidisciplinary care team (e.g., pulmonology, radiology, pathology, and occupational medicine). See also “Diagnostics for ILD” for a broader workup.

Antigen-specific inhalation challenge and specific lymphocyte proliferation testing (i.e., in-vitro assessment of polyclonal T-cell response after antigen exposure) are no longer recommended. [7]

Differential diagnosestoggle arrow icon

The differential diagnoses listed here are not exhaustive.

Treatmenttoggle arrow icon

Antigen avoidance, supportive management of ILD, and specialist-directed pharmacotherapy are the mainstays of therapy. Consider lung transplantation for patients with fibrotic HP that does not respond to medical therapy.

Pharmacotherapy [4]

Complicationstoggle arrow icon

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

Prognosistoggle arrow icon

  • Favorable in the acute stage, but the disease recurs and worsens upon re-exposure
  • Worsens with severity of fibrosis

Referencestoggle arrow icon

  1. Reportable Diseases and Conditions. Updated: January 1, 2011. Accessed: May 5, 2017.
  2. Hypersensitivity Pneumonitis (Extrinsic Allergic Alveolitis). 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. Costabel U, Miyazaki Y, Pardo A, et al. Hypersensitivity pneumonitis. Nat Rev Dis Primers. 2020; 6 (1).doi: 10.1038/s41572-020-0191-z . | Open in Read by QxMD
  5. D'souza RS, Donato A. Hypersensitivity pneumonitis: an overlooked cause of cough and dyspnea.. J Community Hosp Intern Med Perspect. 2017; 7 (2): p.95-99.doi: 10.1080/20009666.2017.1320202 . | Open in Read by QxMD
  6. McGee SR. Evidence-based Physical Diagnosis. Saunders ; 2017
  7. Fernández Pérez ER, Travis WD, Lynch DA, et al. Diagnosis and Evaluation of Hypersensitivity Pneumonitis. Chest. 2021; 160 (2): p.e97-e156.doi: 10.1016/j.chest.2021.03.066 . | Open in Read by QxMD
  8. Raghu G, Remy-Jardin M, Ryerson CJ, et al. Diagnosis of Hypersensitivity Pneumonitis in Adults: An Official ATS/JRS/ALAT Clinical Practice Guideline. Am J Respir Crit Care Med. 2020; 202 (3): p.e36-e69.doi: 10.1164/rccm.202005-2032st . | Open in Read by QxMD
  9. Mikolasch TA, Garthwaite HS, Porter JC. Update in diagnosis and management of interstitial lung disease. Clin Med (Northfield Il). 2017; 17 (2): p.146-153.doi: 10.7861/clinmedicine.17-2-146 . | Open in Read by QxMD
  10. Kalchiem-Dekel O, Galvin J, Burke A, Atamas S, Todd N. Interstitial Lung Disease and Pulmonary Fibrosis: A Practical Approach for General Medicine Physicians with Focus on the Medical History. J Clin Med. 2018; 7 (12): p.476.doi: 10.3390/jcm7120476 . | Open in Read by QxMD
  11. 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
  12. 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

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