• Clinical science

Hypersensitivity pneumonitis (Extrinsic allergic alveolitis)


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.


  • Combined type III and type IV hypersensitivity reaction with genetic predisposition
  • Inhalation of organic particles, primarily through occupational exposure (notifiable occupational disease)
Antigen Source Disease
Animal proteins
Microorganism (s)
  • Actinomycete spores from air conditioners, humidifiers, and water reservoirs
  • Actinomycete spores from moldy hay
  • Actinomycete spores from sugar cane
  • Actinomycete spores from moldy compost
  • Grain weevil dust

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


Clinical features

  • Acute (commencing 4–8 hours after exposure)
    • Flu-like symptoms: Fever, chills, malaise, cough, headache
    • Dyspnea without wheezing
    • Diffuse fine crackles upon auscultation
    • Symptoms subside after 12 hours to several days (in the absence of additional exposure)
  • Chronic (months after continuous exposure)
    • Insidious onset of fatigue, productive cough, progressive dyspnea, cyanosis
    • Bilateral rales
    • Weight loss

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:

  • Positive serology: IgG, IgA, or IgM antibodies
  • Chest x-ray or CT
    • Acute: Patchy reticulonodular or diffuse infiltrates may be found in the mid to upper zone.
    • Chronic : ground-glass attenuation with honeycombing (fibrotic changes) +/- emphysema
  • Pulmonary function test: : restrictive pattern
  • Bronchoalveolar lavage (BAL): : lymphocytic predominance

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



  • Granulomatous inflammation with lymphocytes and polynuclear giant cells


Differential diagnoses


The differential diagnoses listed here are not exhaustive.




  • Respiratory failure
  • Pulmonary heart disease


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