Hypersensitivity pneumonitis (HP), also known as extrinsic allergic alveolitis (EAA), is a delayed hypersensitivity lung disease mediated primarily by cell-mediated and humoral immune responses. It occurs in susceptible individuals after repeated inhalation of organic or inorganic dust antigens. The pathological features include chronic alveolar inflammation predominantly with lymphocyte infiltration, accompanied by cellular bronchiolitis (inflammation centered on the airways) and scattered non-caseating granulomas as characteristic pathological changes.
A classic form of HP is farmer's lung, caused by inhalation of thermophilic actinomycetes or Saccharopolyspora rectivirgula spores from moldy hay. Bird fancier’s lung (e.g., pigeon breeder's lung, parrot breeder's lung) results from inhaling dust containing animal proteins from feathers and droppings. Air-conditioner lung arises from living in environments contaminated with thermophilic actinomycetes from air conditioners or humidifiers. Despite the diverse etiologies, the clinical manifestations of HP are similar and can present as acute, subacute, or chronic forms.
Currently, HP is classified into non-fibrotic HP and fibrotic HP based on imaging and pathological findings.
Non-Fibrotic HP
This typically presents as acute or subacute onset. Acute onset often occurs 4–8 hours after antigen exposure in occupational or domestic environments. Symptoms include chills, fever, general malaise, chest tightness, dyspnea, and cough. If antigen exposure ceases, symptoms can resolve within 24–48 hours. Continued exposure, however, may lead to repeated acute episodes, resulting in gradually progressive dyspnea over weeks or months, accompanied by weight loss, presenting as the subacute form.
Fibrotic HP
This usually results from chronic, long-term exposure to allergens. Repeated inflammation and abnormal repair processes in the lungs lead to fibrosis. The primary manifestations include progressively worsening dyspnea, cough, and weight loss. Inspiratory Velcro crackles may be heard at the lung bases, and a minority of cases may present with clubbing of the fingers.
Diagnosis
A relatively definitive diagnosis can be made based on:
- A clear history of antigen exposure.
- Typical symptom patterns.
- Characteristic findings on HRCT of the chest, such as centrilobular micronodules, patchy ground-glass opacities, and mosaic attenuation due to air trapping.
- Pulmonary fibrosis predominantly in the mid-to-upper lung zones.
- BALF (bronchoalveolar lavage fluid) analysis showing a significant increase in lymphocyte proportion.
Pathological findings from TBLB (transbronchial lung biopsy) or TBLC (transbronchial cryobiopsy) can further support the diagnosis. In atypical cases, surgical lung biopsy may be required.
Treatment
The fundamental treatment for HP is eliminating or avoiding antigen exposure.
For patients with significant pulmonary infiltrates and hypoxemia, corticosteroid therapy can help achieve marked improvement in imaging findings and lung function.