Hypersensitivity pneumonitis

Hypersensitivity pneumonitis (HP) is an inflammation of the alveoli and distal bronchioles caused by an immune response to inhaled allergens.

Occupational exposure to organic dust is the key epidemiological factor - most commonly including Actinomycetes bacteria, animal proteins, or reactive chemicals.

Diagnosis requires a high index of suspicion when evaluating individuals with interstitial lung disease or recurrent flu-like illness.

Immunological reaction to inhaled antigen is corroborative.

No pathognomonic tests.

Treatment involves avoidance of causative agent and use of corticosteroids.

Hypersensitivity pneumonitis (HP), also known as extrinsic allergic alveolitis, is the result of non-IgE mediated immunological inflammation. HP is caused by repeated inhalation of non-human protein, which can be of natural plant or animal origin or can be the result of a chemical conjugated to a human airway protein, such as albumin. The inflammation of HP manifests itself in the alveoli and distal bronchioles. Historically, HP has been categorised as acute, subacute, or chronic, depending on the frequency, duration, and intensity of exposure and the duration of illness.[1]Slavin RG. What the allergist should know about hypersensitivity pneumonitis. Allergy Asthma Proc. 2007 Jan-Feb;28(1):25-7. http://www.ncbi.nlm.nih.gov/pubmed/17390753?tool=bestpractice.com [2]Ismail T, McSharry C, Boyd G. Extrinsic allergic alveolitis. Respirology. 2006 May;11(3):262-8. http://www.ncbi.nlm.nih.gov/pubmed/16635083?tool=bestpractice.com However, these categories are not easily separated. The American Thoracic Society, Japanese Respiratory Society, and Latin American Thoracic Association diagnostic guidelines published in 2020 classify patients as having fibrotic or non-fibrotic HP. Classification is determined by the predominant presence or absence of fibrosis on radiological and/or histopathological examination.[3]Raghu G, Remy-Jardin M, Ryerson CJ, et al. Diagnosis of hypersensitivity pneumonitis in adults. An Official ATS/JRS/ALAT clinical practice guideline. Am J Respir Crit Care Med. 2020 Aug 1;202(3):e36-69. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7397797 http://www.ncbi.nlm.nih.gov/pubmed/32706311?tool=bestpractice.com Many cases of HP are caused by occupational exposure.[4]Quirce S, Vandenplas O, Campo P, et al. Occupational hypersensitivity pneumonitis: an EAACI position paper. Allergy. 2016 Jun;71(6):765-79. http://onlinelibrary.wiley.com/doi/10.1111/all.12866/full http://www.ncbi.nlm.nih.gov/pubmed/26913451?tool=bestpractice.com