Hypersensitivity pneumonitis

Overview 
Theory 
Diagnosis 
Management 
Follow up 
Resources 

Last reviewed: 7 Jun 2025

Last updated: 22 Nov 2023

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.

Various tests may be used to corroborate a presumptive diagnosis, but no test is pathognomonic.

Treatment involves avoidance of causative agent and use of corticosteroids.

Definition

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][2] 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] Many cases of HP are caused by occupational exposure.[4]

History and exam

Key diagnostic factors

exposure to avian protein antigen, especially excrement
mould in work environment
occupational exposure to chemicals
dyspnoea
cough

Full details

Other diagnostic factors

fever/chills
malaise
weight loss/anorexia
bibasilar rales
diffuse rales
clubbing

Full details

Risk factors

smoking
viral infection
exposure to avian protein antigen
exposure to mould antigen
exposure to bacterial antigen
exposure to diisocyanate (e.g., epoxy resin)
exposure to acid anhydride antigen (e.g., paint refinisher)
exposure to metalworking fluid
nitrofurantoin, methotrexate, roxithromycin, and rituximab
e-cigarettes
sertraline
ayurvedic medicine

Full details

Diagnostic investigations

1st investigations to order

chest x-ray
CT chest
serum antigen-specific IgG or IgA
pulmonary function test
diffusing lung capacity of carbon monoxide

Full details

Investigations to consider

bronchoalveolar lavage
lung biopsy

Full details

Treatment algorithm

ACUTE

all patients

Contributors

Authors

Leslie Grammer, MD

Professor

Northwestern University Feinberg School of Medicine

Director

Ernest S. Bazley Asthma and Allergy Center

Clinic Practice Director

Allergy-Immunology

Northwestern Medical Faculty Foundation

Chicago

Disclosures

LG declares that she has no competing interests.

Peer reviewers

Jordan Fink, MD

Professor of Pediatrics

Allergy-Immunology Division

Medical College of Wisconsin

Milwaukee

Disclosures

JF is an author of a number of references cited in this topic.

Alec Chessman, MD

Professor of Family Medicine

Medical University of South Carolina

Charleston

Disclosures

AC declares that he has no competing interests.

Toby M. Maher, MB, MSc, PhD, MRCP

Consultant Respiratory Physician

Interstitial Lung Disease Unit

Royal Brompton Hospital

London

Disclosures

TMM has received consultancy fees from GSK, Actelion, and Philips Respironics. He has received lecture fees from Astra Zeneca and holds an unrestricted industry-academic grant from GSK.

References

Our in-house evidence and editorial teams collaborate with international expert contributors and peer reviewers to ensure that we provide access to the most clinically relevant information possible.

Key articles

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.Full text Abstract

Quirce S, Vandenplas O, Campo P, et al. Occupational hypersensitivity pneumonitis: an EAACI position paper. Allergy. 2016 Jun;71(6):765-79.Full text Abstract

Fernández Pérez ER, Travis WD, Lynch DA, et al. Diagnosis and evaluation of hypersensitivity pneumonitis: CHEST guideline and expert panel report. Chest. 2021 Aug;160(2):e97-156.Full text Abstract

Reference articles

A full list of sources referenced in this topic is available to users with access to all of BMJ Best Practice.

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Hypersensitivity pneumonitis

Summary

Definition

History and exam

Key diagnostic factors

Other diagnostic factors

Risk factors

Diagnostic investigations

1st investigations to order

Investigations to consider

Treatment algorithm

all patients

Contributors

Authors

Leslie Grammer, MD

Disclosures

Peer reviewers

Jordan Fink, MD

Disclosures

Alec Chessman, MD

Disclosures

Toby M. Maher, MB, MSc, PhD, MRCP

Disclosures

References

Key articles

Reference articles

A full list of sources referenced in this topic is available to users with access to all of BMJ Best Practice.

Differentials

Guidelines

Videos

Log in or subscribe to access all of BMJ Best Practice

Log in or subscribe to access all of BMJ Best Practice

Log in to access all of BMJ Best Practice