Current concepts in the diagnosis and treatment of hypersensitivity pneumonitis: a narrative review.

Hypersensitivity pneumonitis (HP) is an interstitial lung disease triggered by inhalation of a variety of environmental antigens. HP ranges from acute manifestations to progressive fibrosis and can be classified as nonfibrotic or fibrotic based on imaging and histopathological features. The pathophysiological mechanism of HP involves an exaggerated immune cell response and, in chronic stages, fibroblast activation leading to fibrosis. Diagnosis requires integrated assessment of exposure history, HRCT findings, BAL findings, and histopathological findings, as well as multidisciplinary discussion. In nonfibrotic HP patients, HRCT typically shows ground-glass opacities, centrilobular nodules, and mosaic attenuation, whereas, in fibrotic HP patients, HRCT often shows reticulation, traction bronchiectasis, and honeycombing. Typical histological features include cellular bronchiolitis, poorly formed granulomas, and interstitial fibrosis. Recent advances in imaging-including artificial intelligence-based pattern recognition-and investigation of biomarkers are improving precision and will likely contribute to the diagnostic investigation of HP. Management strategies depend on disease phenotype and severity, with antigen avoidance remaining the cornerstone of therapy. Immunomodulators such as corticosteroids, azathioprine, and mycophenolate may be used when there is evidence or suspicion of active inflammatory disease, whereas nintedanib is indicated for progressive fibrotic forms. Emerging phosphodiesterase 4B inhibitors have shown promise based on recent evidence. A greater number of comorbidities are observed in patients with fibrotic HP and are associated with worse survival, particularly pulmonary hypertension. Understanding genetic predisposition, comorbidities, and the transition from inflammation to fibrosis is essential to guide personalized treatment and improve outcomes. This review provides an update of pathophysiology, current diagnostic approaches, and therapeutic options, including emerging strategies.