title: Pathology of hypersensitivity pneumonitis.
abstract: OBJECTIVE
  Hypersensitity pneumonitis, caused by inhalation of various antigens, is characterized by interstitial mononuclear cell infiltration, nonnecrotizing granulomas, cellular bronchiolitis, and fibrosis. The pathological picture of chronic hypersensitivity pneumonitis is, however, complicated; it is sometimes difficult to differentiate chronic hypersensitivity pneumonitis from idiopathic pulmonary fibrosis/usual interstitial pneumonia, nonspecific interstitial pneumonia, and connective-tissue-related lung disease. The clinical, radiological, and pathological features of chronic hypersensitivity pneumonitis have recently been described. This study reviews the previously reported information and provides new insights into the pathological features of chronic hypersensitivity pneumonitis.
  
  
  RESULTS
  The pathological features of chronic hypersensitivity pneumonitis comprise overlapping usual interstitial pneumonia-like pattern with subpleural patchy fibrosis, alternating normal alveoli and fibroblastic foci, a nonspecific interstitial pneumonia-like pattern, and centrilobular fibrosis. In contrast to pathological features of acute and subacute hypersensitivity pneumonitis, epithelioid cell granulomas are sparse or absent, but giant cells are seen in the interstitium. Bridging fibrosis between peribronchiolar area and perilobular areas is an outstanding feature of chronic hypersensitivity pneumonitis. Autopsy cases of chronic hypersensitivity pneumonitis have demonstrated not only upper lobe contraction but also lower lobe contraction, mimicking usual interstitial pneumonia pattern and diffuse alveolar damage.
  
  
  CONCLUSIONS
  The present review focuses on the pathological features of chronic hypersensitivity pneumonitis and presents that centrilobular fibrosis and bridging fibrosis are the important hallmarks of chronic hypersensitivity pneumonitis, even with a usual interstitial pneumonia-like pattern.
authors.affiliation: Department of Pathology, Japanese Red Cross Medical Center, Japan. byori@med.jrc.or.jp
authors.forename: Tamiko
authors.initials: T
authors.lastname: Takemura
authors.forename: Takumi
authors.initials: T
authors.lastname: Akashi
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Ohtani
authors.forename: Naohiko
authors.initials: N
authors.lastname: Inase
authors.forename: Yasuyuki
authors.initials: Y
authors.lastname: Yoshizawa
mesh_terms: D000542:Alveolitis, Extrinsic Allergic / Q000175:diagnosis* / Q000473:pathology*; D001706:Biopsy; D002908:Chronic Disease; D003240:Connective Tissue Diseases / Q000175:diagnosis / Q000473:pathology; D003937:Diagnosis, Differential; D006801:Humans; D008168:Lung / Q000473:pathology; D017563:Lung Diseases, Interstitial / Q000175:diagnosis / Q000473:pathology; D011658:Pulmonary Fibrosis / Q000175:diagnosis / Q000473:pathology