What are the key clinical features that differentiate hypersensitivity pneumonitis from other forms of interstitial lung disease?

Key clinical features that differentiate hypersensitivity pneumonitis (HP) from other interstitial lung diseases (ILDs) include a history of exposure to known environmental antigens, acute onset of symptoms following exposure, and characteristic radiological and immunological findings.

HP typically presents with a history of exposure to organic antigens such as bird proteins, mold, or farming dust, which is less common in other ILDs ¹ (Pitcher, 1990). The clinical presentation often includes acute or subacute symptoms such as cough, dyspnoea, fever, and malaise occurring hours after antigen exposure, contrasting with the more insidious onset seen in idiopathic pulmonary fibrosis and other ILDs ¹ (Churg, 2022).

On examination, inspiratory crackles may be present but are not specific; however, the temporal relationship to antigen exposure is a key differentiator ¹ (Yang et al., 2022). Radiologically, high-resolution CT scans in HP often show a combination of ground-glass opacities, centrilobular nodules, and mosaic attenuation due to air trapping, which differ from the usual interstitial pneumonia pattern seen in idiopathic pulmonary fibrosis ¹ (Churg, 2022). Additionally, bronchoalveolar lavage typically reveals lymphocytosis in HP, which is less prominent in other ILDs ¹ (Yang et al., 2022).

Immunologically, the presence of serum precipitating antibodies against suspected antigens supports the diagnosis of HP but is not diagnostic alone ¹ (Pitcher, 1990). Importantly, the clinical course of HP may improve with antigen avoidance, a feature not seen in many other ILDs ¹ (Churg, 2022).