What Is Extrinsic Allergic Alveolitis

Extrinsic allergic alveolitis, also known as hypersensitivity pneumonitis is an allergic reaction in the lungs due to certain inhaled material in the environment. The symptoms can range from acute symptoms like fever and cough to a prolonged illness causing lung functions to decline. Not everyone is susceptible to these various allergens that range from fungal organisms to bird droppings to chemicals. Treatment depends on whether a person is having an acute illness or long term disease.1

If you have ever heard of conditions called ‘bird fancier’s lung’, ‘hot tub lung’ and ‘farmer’s lung’, they all mean the same condition, which is extrinsic allergic alveolitis. All those names actually indicate the association with the different environmental triggers for the same condition. Overall, extrinsic allergic alveolitis has three patterns of presentation. One is an acute illness where after a heavy exposure to the allergen, a person gets symptoms including fever and cough that resolve within several hours following supportive treatment and removal of exposure to allergen.

The other is a subacute illness presenting with difficulty in breathing, fever and cough over weeks and settling with medication like steroids. The chronic type is a prolonged illness exhibiting previously mentioned symptoms along with weight loss, lung scarring and declining lung functions.1

The lungs and extrinsic allergic alveolitis

Lungs are generally thought of as a pair of balloons that inflate and deflate as we breathe andhey comprise millions of tiny air filled sacs called alveoli. ‘Extrinsic’ means outside of the body and ‘alveolitis’ means inflammatory reaction in the alveoli which can be caused by the inhalation of an ‘outside’ or environmental allergen. The allergen particles have to be less than 5 microns to be able to reach the alveoli. Additionally the inhalation of allergens sets off an immune response in the body causing various other systemic symptoms.2

The body and extrinsic allergic alveolitis

For extrinsic allergic alveolitis to occur, prolonged or intense exposure to the allergen must happen. Then too, not all individuals go on to develop this immune reaction. Although some people are apparently susceptible to developing extrinsic allergic alveolitis, no definitive underlying genetic factor has yet been identified.3

This disease is mainly driven by the body’s immunological response. This means the allergen in the lung triggers the body’s defense system and the body mounts an immune response.

There are four main types of immune reactions and extrinsic allergic alveolitis showcases type III and type IV hypersensitivity depending on the form presented.3

Causes of extrinsic allergic alveolitis 

There are numerous environmental allergens that cause this disease and these are broadly categorized into three groups - microbes, animal proteins, chemicals like isocyanate, some medicines and metals like zinc.4

Examples of allergens and their associated names are as follows.2

Allergen SourceExtrinsic Allergic Alveolitis Disease Name
Bird droppingsBird Fancier's Lung
Mould sporesFarmer's Lung
Bacteria from humidifiersHumidifier Lung
Mouldy hay or strawMalt Worker's Lung
Air conditioner systemsAir Conditioner or Ventilation System Lung
Wood dustWoodworker's Lung
Grain dustGrain Handler's Lung
Pigeon droppingsPigeon Breeder's Lung
Mushroom compostMushroom Worker's Lung
Chemicals in workplaceChemical Worker's Lung

Signs and symptoms of extrinsic allergic alveolitis

The disease is classified into three categories based on symptoms and duration.

The clinical features of acute, subacute and chronic hypersensitivity pneumonitis are presented.1 

SymptomsLung FunctionDuration
AcuteFever, cough, difficulty in breathing, and crackling sounds may be heard when listening to the lung with a stethoscopeLung function is normally not affectedSymptoms occur after few hours and can resolve after several hours after allergen removal 
Subacute Fever, cough, difficulty in breathing, fatigue and crackling sounds may be heard when listening to the lung with a stethoscopeLung function may be normalProgresses slowly and may last from weeks or months
Chronic Fever, cough, difficulty in breathing, fatigue, weight loss, and fine crackling sounds may be heard when listening to the lung with a stethoscope Symptoms occur after a few hours and can resolve after several hours after allergen removal Slowly progressive respiratory failure lasting over years

Diagnosis of extrinsic allergic alveolitis

Diagnosing extrinsic allergic alveolitis involves understanding the symptoms and their association with the environmental triggers. A doctor will often take a detailed history including the patient’s occupation and possible exposure to known environmental triggers. Physical examination will not reveal much except on chest examination where the doctor will auscultate the lungs and hear crackles in the lung. The investigations that are needed include imaging studies like chest x-rays and CT scans, blood tests, lung function tests2 and sometimes lung biopsies.1

  • Chest x-ray – will often not reveal much although it may show some generalized lung shadows
  • A high-resolution CT scan – shows changes in the lung including dilated air spaces and scarring. There is also a type of appearance called ‘ground glass appearance’ that is characteristically seen in CT of the chest
  • Cytology – some cells of the airways can be sampled by what is known as bronchoalveolar lavage/washings. This procedure tests cells lining the airways that are collected as washings from airways. These can show a predominance of inflammatory cells in extrinsic allergic alveolitis
  • Biopsy – a sampling of the lung, known as lung biopsy is sometimes done, usually to differentiate this disease from other types of chronic and scarring lung diseases called interstitial lung diseases. The biopsy will then be checked under the microscope for changes. These can show scarring of lung tissue as well as collections of inflammatory cells mainly surrounding the airways
  • Blood tests – these may sometimes show an increase in certain antibodies in the blood that show up when there is an immune reaction happening in the body.5 There are other blood tests that will show a general inflammatory process in the body called ESR and CRP which will show an increase in their levels
  • Lung function tests - These tests are mostly used in the chronic disease process of extrinsic allergic alveolitis. These tests will estimate the function of lungs including its ability to take in oxygen. Some other chronic lung diseases can also show this pattern

Treatment and management of extrinsic allergic alveolitis 

Treating extrinsic allergic alveolitis involves a combination of steps to relieve symptoms and prevent further lung damage. 

Removal of exposure to allergen

The cornerstone of treatment at any stage of the disease is to remove the allergen from the patient’s environment or avoid exposure to it. This might mean making changes in the living or working environment. However in the long term stage of the disease, removal of allergen will not reverse the progression of the disease.5


Steroids have been reported to provide some benefit in the acute and subacute types of extrinsic allergic alveolitis. Steroids are capable of reducing the inflammatory reaction of the body, causing a reduction of symptoms that occur due to the inflammation.

Steroids have not shown much benefit for the chronic type of extrinsic allergic alveolitis, especially with regard to the outcome of the disease.5

Supportive treatment and lung transplant

In severe or chronic disease, oxygen therapy may be indicated as a supportive measure. The definitive treatment for the chronic disease type of extrinsic allergic alveolitis is a lung transplant.5

What are the new research findings about extrinsic allergic alveolitis?

Some genetic factors and environmental factors contributing to the susceptibility of a person to this disease have been reported. These include the age and an individual’s immune type called HLA typing as well as, smoking, loft ventilation and certain viral infections.5

Living with extrinsic allergic alveolitis

Living with extrinsic allergic alveolitis can present challenges, but with proper management and awareness, a fulfilling life is still possible. Learning about the specific triggers that worsen your symptoms is crucial, as avoiding these allergens can greatly improve your quality of life. This might involve making changes to the living environment, using protective measures, and ensuring proper ventilation. It's also important to follow the doctor's advice regarding medications and treatment plans. 


Extrinsic allergic alveolitis is a condition that starts in the lungs due to inhaled environmental allergens and causes an immune reaction in the body. This is a condition that has a variable course, with either an acute flu-like illness or prolonged difficulty in breathing that leads to failure of lung functions. Not everyone is susceptible to these environmental allergens but those who fall victim to the disease often get exposed to the allergen heavily or for prolonged periods. Numerous allergens have been implicated, ranging from bird droppings to fungal spores to chemicals.

Investigations such as chest x-rays, CT scans, blood tests, lung function tests and biopsies may be needed to diagnose the condition. Any treatment begins with the removal of the allergen exposure, and the remaining course depends on the presentation of symptoms and long-term effects on the lungs. Acute illness is usually treated with supportive treatment and usually resolves when the allergen is removed. Subacute illness may need treatment like steroids. Long-term illness leads to respiratory failure, and treatment includes supportive measures like oxygen therapy. A definitive treatment and last resort will be a lung transplant. Early diagnosis of this condition is essential to better treatment and disease outcome.


  1. Hypersensitivity pneumonitis [Internet]. www.pathologyoutlines.com. [cited 2023 Aug 26]. Available from: https://www.pathologyoutlines.com/topic/lungnontumorextrinsicallergic.html
  2. Government of Canada CC for OH and S. Hypersensitivity Pneumonitis (Extrinsic Allergic Alveolitis): OSH Answers [Internet]. www.ccohs.ca. 2020. Available from: https://www.ccohs.ca/oshanswers/diseases/alveolitis.html
  3. Hypersensitivity Pneumonitis: Background, Pathophysiology, Etiology. eMedicine [Internet]. 2020 Nov 25; Available from: https://emedicine.medscape.com/article/299174-overview
  4. Yuranga Weerakkody. Hypersensitivity pneumonitis | Radiology Reference Article | Radiopaedia.org [Internet]. Radiopaedia.org. 2019. Available from: https://radiopaedia.org/articles/hypersensitivity-pneumonitis?lang=gb
  5. Ismail T, McSharry C, Boyd G. Extrinsic allergic alveolitis. Respirology. 2006 May;11(3):262–8.
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Varuni Upamali Fernando

MBBS(Colombo), DipRCpath, CHCCT(UK)

Curent role as Specialty Doctor in Histopathology and previously as Associate Specialist in GI pathology. STEM ambassador and former freelance copywriter for advertising agencies and healthcare institutes.

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