Hypersensitivity pneumonitis

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. The clinical manifestations of HP depend on the concentration and frequency of exposure. The clinical syndromes - acute, sub-acute, and chronic HP - present differently.[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 Some authors are now reporting that there are only two clinical syndromes; however, they vary as to whether they group sub-acute with the acute or the chronic form.[3]Miller R, Allen TC, Barrios RJ, et al. Hypersensitivity pneumonitis: a perspective from members of the Pulmonary Pathology Society. Arch Pathol Lab Med. 2017 Jun 14 [Epub ahead of print]. http://www.archivesofpathology.org/doi/pdf/10.5858/arpa.2017-0138-SA http://www.ncbi.nlm.nih.gov/pubmed/28613913?tool=bestpractice.com [4]Churg A, Bilawich A, Wright JL. Pathology of chronic hypersensitivity pneumonitis: what is it? What are the diagnostic criteria? Why do we care? Arch Pathol Lab Med. 2017 May 24 [Epub ahead of print]. http://www.archivesofpathology.org/doi/pdf/10.5858/arpa.2017-0173-RA http://www.ncbi.nlm.nih.gov/pubmed/28537805?tool=bestpractice.com Many cases of HP are caused by occupational exposure.[5]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