17th January 2024

Background

Epidemiology

Hypersensitivity pneumonitis (or extrinsic allergic alveolitis) is an immune-mediated respiratory syndrome caused by allergen inhalation and subsequent reaction. This leads to alveolar and bronchiole inflammation via a non-IgE mediated process. The most common allergen is organic dust; however, there are hundreds of known causative agents, so identifying the allergen can be difficult in some cases. Pigeon fancier’s lung and Farmer’s lung are examples of hypersensitivity pneumonitis caused by pigeon exposure (avian protein antigen in faeces) and grain/straw mould (usually due to Aspergillus, Alternaria or Trichosporon fungi) respectively.

Prevalence and incidence of the condition overall are thought to be low; however, mild cases can masquerade as other conditions, such as viral illness and, therefore, remain undetected for a long period of time. Smoking may be a protective factor against developing acute HP; some evidence suggests that smokers exposed to causative allergens form fewer antibodies against antigens present and, therefore, do not manifest a clinical presentation of HP. However, in cases of established disease, smoking may influence a more severe presentation and cause recurrent episodes of exacerbation.

Adapted from: King Jr, T. (2021) Hypersensitivity pneumonitis (extrinsic allergic alveolitis): Epidemiology, causes, and pathogenesis, UpToDate. Edited by K. Flaherty and P. Dieffenbach. UpToDate. Available at: https://www.uptodate.com/contents/hypersensitivity-pneumonitis-extrinsic-allergic-alveolitis-epidemiology-causes-and-pathogenesis?search=hypersensitivity+pneumonitis&source=search_result&selectedTitle=1~150&usage_type=default&display_rank=1 (Accessed: January 15, 2023).