Hypersensitivity Pneumonitis (Extrinsic Allergic Alveolitis)


  • Results from the inhalation of organic antigens which cause a diffuse immune complex reaction in the alveoli and bronchioles
    • E.g. Farmer’s lung (hay- mycropolyspora faeni (or Saccharopolyspora rectivirgula); rarely aspergillus fumigatus); Bird Fancier’s lung (bird droppings/proteins)
    • Other causes include drugs e.g. Flecainide, Gold, Nitrofurantoin, Amiodarone, cytotoxic agents e.g. Bleomycin, sulfasalazine


  • Consistent with both type III (immune complex deposition) hypersensitivity reaction BUT ALSO a type IV (cell mediated) hypersensitivity reaction.  It is now thought that a cellular reaction is more important and more likely to cause HP
    • Presence of both immune complexes and non-caseating granulomas
    • Inflammation is predominantly peribronchiolar (cf UIP)
    • Chronic forms can be accompanied by fibrosis

Clinical Presentation

  • Often flu-like symptoms with cough, breathlessness and wheeze which usually occurs several hours after exposure (particularly if there is a large exposure).  If there is low level, chronic exposure, the patient may present with slowly progressive breathlessness
  • Widespread end-inspiratory crackles/squeaks is the classical finding


  • CXR- may show ill-defined patchy shadowing (can be confused with pneumonia)
  • HRCT- bilateral ground glass shadowing and areas of consolidation superimposed on small centrilobular nodular opacities with an upper-middle lobe predominance
    • Features of fibrosis may be seen in chronic cases (volume loss, linear opacities, architecture loss
  • PFTs- Restrictive lung function; reduced lung volume and gas transfer
  • Where HP is suspected by cause unknown:
    • ?Visit patient’s home/workplace
    • BAL may show increased CD8+ lymphocytes
    • Biopsy can confirm diagnosis


  • Acute
    • Steroids (oral) – usually 40mg prednisolone per day
    • Oxygen therapy if required
  • Avoid exposure (use of protection e.g. masks, if patient is unable to avoid the exposure)

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