Pathology of hypersensitivity pneumonitis

Area of Science:

• Pulmonary Medicine
• Pathology
• Immunology

Background:

• Hypersensitivity pneumonitis (HP) results from antigen inhalation, causing lung inflammation.
• Chronic HP (CHP) diagnosis is challenging due to overlapping features with idiopathic pulmonary fibrosis (IPF)/usual interstitial pneumonia (UIP), nonspecific interstitial pneumonia (NSIP), and connective tissue disease-associated lung disease.

Purpose of the Study:

• To review and provide new insights into the pathological features of chronic hypersensitivity pneumonitis.
• To differentiate CHP from other interstitial lung diseases based on pathological findings.

Main Methods:

• Review of previously reported clinical, radiological, and pathological information on chronic hypersensitivity pneumonitis.
• Analysis of pathological features, including fibrosis patterns, granuloma presence, and giant cells.

Main Results:

• CHP exhibits UIP-like patterns with subpleural patchy fibrosis, normal alveoli, fibroblastic foci, NSIP-like patterns, and centrilobular fibrosis.
• Epithelioid granulomas are infrequent in CHP, unlike acute/subacute HP, but giant cells are present interstitially.
• Bridging fibrosis and lobar contraction (upper and lower) are characteristic findings in CHP, sometimes mimicking UIP or diffuse alveolar damage.

Conclusions:

• Centrilobular and bridging fibrosis are crucial diagnostic hallmarks of chronic hypersensitivity pneumonitis.
• These fibrotic patterns are important for differentiating CHP, even when presenting with a UIP-like appearance.