99. Ask a Fellow- Hypersensitivity Pneumonitis

Hypersensitivity pneumonitis (HP) is an immunologic interstitial lung disease triggered by inhaled antigens, necessitating a meticulous clinical history to identify occupational or hobby-related exposures. Respirology fellow Dr. Dan Gillet distinguishes between non-fibrotic and fibrotic phenotypes, which dictate diagnostic and management pathways. Diagnosis relies on identifying the offending agent, characteristic high-resolution CT findings, and histopathologic evidence, often involving bronchoalveolar lavage (BAL) to assess lymphocytosis. While antigen avoidance remains the primary treatment, symptomatic patients with physiologic impairment typically require corticosteroid therapy. Refractory cases may necessitate second-line immunosuppressants like mycophenolate mofetil or azathioprine. Although most acute cases achieve near-total recovery, chronic fibrotic HP carries a more complex prognosis, occasionally requiring lung transplantation. This clinical framework emphasizes early recognition and systematic evaluation to prevent irreversible pulmonary fibrosis.