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Hypersensitivity pneumonitis – Causes and Symptoms of Hypersensitivity Pneumonitis

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What is Hypersensitivity pneumonitis ?

Hypersensitivity pneumonitis (HP) is caused by certain environmental allergens that cause allergic reaction leading to inflammation of the lung tissues (alveoli and small airways) resulting in difficulty in breathing and in prolonged cases it causes irreversible lung scarring. It is also called extrinsic allergic alveolitis. In this article you will know about causes and symptoms of hypersensitivity pneumonitis.

An inhaled antigen causes sensitization which is manifested by specific circulating IgG antibodies but sensitization alone is not sufficient as a defining characteristic.

How does it all happen?

HP is an immune-mediated, type IV hypersensitivity reaction, condition which occurs due to repetitive exposure to allergens that are small enough to deposit in distal airways and alveoli causing acute neutrophilic and mononuclear alveolitis which is followed by interstitial lymphocytic infiltration (TH1 & TH 17) and granulomatous reaction resulting in interstitial fibrosis with bronchiolar obliteration on prolonged exposure.

Initially cigarette smoking has shown to delay or prevent HP by decreasing the lung’s immune response to the allergens but once the disease has occurred cigarette smoking has shown to exacerbate the condition.

Causes of hypersensitivity pneumonitis:

Environmental stimuli: smoking, being the most common cause. Passive smoking also results in being a cause. Then environmental factors like bird droppings, animal fur, bird feathers, fungus growing in ventilation systems and humidifiers.

Occupational hazards: chemical workers, farmers, animal & bird breeders, professionals cleaning air conditioners and humidifiers.


Read More hypersensitivity pneumonitis diagnosis


Below is table showing allergens, their source and disease they cause.




Risk factors:

• Age: though the disease can occur at any age, many patients tend to get diagnosed around 50-55 years of age, in which chronic interstitial lung disease is the commonest type in children.

• Genetics: its responsible for a strong immune reaction in certain individuals whereas in others with family history of HP or pulmonary fibrosis can lead to risk of development of HP and presents as familial hypersensitivity pneumonitis when it occurs in a relative.

• Occupation: farmers, woodworkers, wine makers and birds/animals breeders who have long term of exposure to the allergens hence have a higher risk of developing HP.

• Life style: smoking has shown to exacerbate the condition once it has been established.

• Viral infections: They may increase the risk of developing HP in presence of allergens.

Symptoms of hypersensitivity pneumonitis:

HP has been categorized into acute, subacute and chronic forms.

Acute HP occurs within 4-8 hours following the exposure with symptoms being intense and show resolution within few hours to days that is if no further exposure to the allergens takes place.

• Fever

• Chills

• Malaise

• Dyspnoea

• Dry cough

• Chest tightness

Subacute HP results from an ongoing antigen exposure where onset of respiratory and systemic symptoms of hypersensitivity pneumonitis are more gradual over period of weeks time. It shows resolution slower than acute phase on removal of antigen, from weeks to months. It may present as culmination of intermittent episodes of acute HP.

Chronic HP presents with more gradual onset and is more progressive with severe symptoms of hypersensitivity pneumonitis of

• Cough

• Fatigue

• Weight loss

• Shortness of breath, especially with exertion or activity

There can be irreversible component to the respiratory symptoms of hypersensitivity pneumonitis even when the antigen is removed due to permanent lung damage (lung fibrosis) which cause hypoxemic (lack of oxygen) respiratory failure.

Alarming signs & symptoms of hypersensitivity pneumonitis:

• Shallow and fast breathing

• Weight loss

• Clubbing of digits (they become wide and rounded)

• Dyspnea with exertional fatigue (right heart failure)


Read More – Home Remedies for Sinusitis Pain (Chronic Sinusitis)



• Pulmonary hypertension results due to damage to the pulmonary blood vessels.

• Heart failure is a serious complication that occurs due to inflammation resulting in burdening the heart which fails to pump enough blood.

• Pulmonary fibrosis cause irreversible lung parenchyma damage resulting in permanently damage to functioning of the lung reducing its ability to oxygenate the blood.


The diagnosis of

• Lung function testSpirometer devices are used to assess the pulmonary functioning in form of how long the lung can hold, take in and move out the air (less air in – restrictive type; less air out- obstructive type of lung disease). It is helpful in assessment of the ongoing treatment and in grading the seriousness the disease. It helps in characterization of physiological impairment and knowing diffusion capacity for carbon monoxide in cases with lung fibrosis.


• Avoid cigarette smoke. Quit cigarette smoking and avoid being a passive smoker as well, since it exacerbates the condition.

• Wear a mask (N95 respirators) to avoid environmental stimuli like birds, feathers, pollution , dust, fire smokes, to be also used by farmers, woodworkers, wine makers, animal and bird breeders.

• Maintaining hygiene: simple steps like washing hands, use of hand sanitizers , and wearing a mask can prevent from exposure to the allergens to some extent.

• Flu shots: Getting flu vaccine can prevent from infections affecting the lung.

• Cleaning: deep cleaning the ventilation system, timely serving of AC- conditioners/ humidifiers and cleaning out the house and carpets of the dust, animal fur/ bird feathers .


• Remove, replace, change and stay away from the antigen/allergen, shift to new place or job if required.

• Cessation of smoking as it helps is stopping the condition from exacerbating.

Living with HP

• Don’t skip doctor’s appointment in order to receive right treatment and to keep a tab on all new and old symptoms in order to assess the progression of the disease.

• Vaccination: Go in for Flu shots to prevent from getting any infection.

• Start with Pulmonary rehabilitation after consulting your doctor.

• Quit smoking as it exacerbates the condition.

• Keep your environment (inside and outside of your house)clean of stagnant water since its a good breeding place for bacteria and fungi.

• Reduce dust in your home, work place. Clean carpets, curtains regularly to make them dust free. Invest in a good vacuum cleaner.

• Avoid humidity below 60 % at your work and home.

• Air purifying respirators are good options especially for people who are exposed at work.

• Invest in getting Electrostatic dust filter in the return ducts of central air conditioning systems since they help in reducing dust.

• Properly dry and store farm products if you work with them.

• Clean ventilating systems

• Use air purifiers


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