Hypersensitivity pneumonitis (HP) is traditionally divided on clinical grounds into acute,
subacute, and chronic stages. Most biopsy specimens come from patients in the subacute
stage, in which there is a relatively mild, usually peribronchiolar, chronic interstitial
inflammatory infiltrate, accompanied in most cases by poorly formed interstitial granulomas
or isolated giant cells. However, the pathologic features in the chronic, ie, fibrotic stage, are
poorly defined in the literature. These features are important to recognize because the …