Hypersensitivity pneumonitis (HP)

Author: V. Dimov, M.D., Allergist/Immunologist and Assistant Professor at University of Chicago
Reviewer: S. Randhawa, M.D., Allergist/Immunologist and Assistant Professor at NSU


Hypersensitivity pneumonitis is a pulmonary inflammatory condition caused by immunologic sensitization to antigens. Prevalence varies between 0.4-5% of dairy farmers (Farmer’s lung) to 8% of pigeon breeders.

Mind map of hypersensitivity pneumonitis (HP) (click to enlarge the image).

Pathophysiology of HP

Lymphocytes and macrophages play important roles in the inflammatory response.

T cell predominance in lung diseases:

CD8: hypersensitivity pneumonitis (HP) and COPD
Th2, CD4: Asthma, ABPA, pulmonary eosinophilia
Th1, CD4: granulomatous TB, sarcoid, berylliosis

Clinical Features of HP

The acute form begins 4-8 hours after antigen exposure and is characterized by chills, fever, and SOB. It is not accompanied by extrapulmonary manifestations as splenomegaly, lymphadenopathy, or arthritis. For example, finger clubbing is very unusual in HP and should prompt the search for an alternative diagnosis.

Chronic hypersensitivity pneumonitis develops insidiously over months and presents with SOB, fatigue, and weight loss.

Diagnosis of HP

Diagnostic criteria for hypersensitivity pneumonitis include:

1. interstitial lung disease based on history, physical examination or PFTs
2. radiographic findings
3. exposure to a causative agent
4. serum precipitating antibodies to causative antigens

Additional criteria include:

- BAL fluid lymphocytosis with CD8+ cells
- lung biopsy findings
- symptoms and PFTs changes after re-exposure to causative agent

CXR is normal in 10% of and in these cases a HRCT is needed.

PFTs show a restrictive pattern, with decreased DLCO, FVC, and total lung capacity.

Lung biopsy may be needed if diagnosis is uncertain. Characteristic histologic findings infiltration with lymphocytes and plasma cells (100%), noncaseating granulomas (70%), interstitial fibrosis (65%) and bronchiolitis (50%). Granulomas in the walls of blood vessels, found in patients with sarcoidosis, are absent in hypersensitivity pneumonitis.

A normal CXR does not exclude hypersensitivity pneumonitis. Measurement of serum precipitating antibodies and BAL findings are not specific to confirm a diagnosis of hypersensitivity pneumonitis. Lung biopsy is rarely needed.

Laboratory findings in HP

- Elevated WBC, eosinophils, Igs (except IgE), LDH
- Serum IgG precipitins
- BAL shows lymphocytosis, elevated CD8 and very elevated CD4 counts. The ration of CD4/CD8 is less than

Causative Agents of HP

The relevant antigen to hypersensitivity pneumonitis cannot be identified in up to 20% to 30% of patients http://buff.ly/SJejqa

Farmer's Lung and Bird-Breeder's Lung are the most common.

Causes of hypersensitivity pneumonitis (HP) (click to enlarge the image).

Bird-Breeder's Lung, also called Bird-Fancier's Lung, Pigeon-Breeder's Lung, and Poultry-Worker's Lung. Caused by avian proteins. Exposure is from feathers and bird droppings.

Farmer's Lung. Caused by the molds Thermophilic actinomycetes, Saccharopolyspora rectivirgula. Exposure is generally from moldy hay but may be found elsewhere.

Bagassosis. Caused by Thermophilic actinomycetes. Exposure is from moldy bagasse (pressed sugar cane).

Malt Worker's Lung. Caused by Aspergillus clavatus. Exposure is from moldy barley.

Humidifier Lung. Caused by the T. candidus, Bacillus subtilis, B. cereus, and Klebsiella oxytoca; the fungus Aureobasidium pullulans; and the amoebae Naegleria gruberi, Acanthamoeba polyhaga, and Acanthamoeba castellani. Exposure is from mist from standing water.

Mushroom Worker's Lung. Caused by Thermophilic actinomycetes. Exposure is from mushroom compost.

Compost Lung. Caused by Aspergillus. Exposure is from compost.

Peat Moss Worker's Lung. Caused by Monocillium sp. and Penicillium citreonigrum. Exposure if from peat moss.

Suberosis. Caused by Penicillum frequentans. Exposure is from moldy cork dust.

Japanese Summer-Type HP. Caused by Trichosporon cutaneum. Exposure is from damp wood and mats.

Cheese-Washer's Lung. Caused by Pencillum casei or P. roqueforti. Exposure is from cheese casings.

Mnemonic for Cheese washer's lung

Cheese washer's lung (moldy cheese)
Casei, Penicillium casei
Clavatus, Aspergillus clavatus

Metalworking Fluids HP. Caused by Nontuberculous Mycobacteria. Exposure is from mist from metalworking fluids.

Hot Tub Lung. Caused by Mycobacterium avium complex. Exposure is from mist from hot tubs.

Mollusc Shell HP. Caused by aquatic animal proteins. Exposure is from mollusc shell dust.

Isocyanate HP. Caused by TDI, HDI, and MDI. Exposure is from paints, resins, and polyurethane foams.

TMA HP. Caused by Trimellitic anhydride. Exposure is from plastics, resins, and paints.

Beryllium. Caused by Beryllium. Exposure in the electronics industry.

Wine-grower's lung. From Botrytis cinerea mold on grapes.

List of causes based on Hypersensitivity pneumonitis, Wikipedia.

Drugs that can cause HP include:

- Amiodarone
- Cyclosporine
- Gold
- Minocycline
- Nitrofurantoin
- Methotrexate
- Heroin

Differential diagnosis of HP

- Organic dust toxic syndrome - this is a noninfectious, febrile illness that occurs after exposure to dust contaminated by bacteria or fungal spores. This condition is much more common than HP and typically occurs during the summer months.

- Humidifier fever is a toxic alveolitis due to inhalation of water with endotoxin.

- Silo unloaders disease due to exposure to NO2. The exposure can lead to asphyxia, e.g. farmer entering a silo.

- Usual interstitial pneumonia (UIP) - clubbing is present and neutrophils are predominant in BAL.

Treatment and Prevention of HP

The prognosis is excellent if the exposure to antigen is brief (less than 6 months), exposure to antigen is stopped.


Allergy and Immunology MKSAP, 3rd edition.
Sarcoidosis is a Th1/Th17 multisystem disorder, Thorax 2011.
Management of hypersensivity pneumonitis - free full text review, PDF http://buff.ly/WOywMT

Published: 02/08/2008
Updated: 01/14/2011

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