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Hypersensitivity Pneumonitis - Causes, Symptoms, Diagnosis, and Treatment

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Hypersensitivity Pneumonitis - Causes, Symptoms, Diagnosis, and Treatment

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Hypersensitivity pneumonitis causes an inflammatory lung. Read the article to learn about the symptoms, diagnosis, and treatment of hypersensitivity pneumonitis.

Written by

Dr. Saberitha

Medically reviewed by

Dr. Kaushal Bhavsar

Published At July 27, 2022
Reviewed AtJune 21, 2024


Hypersensitivity pneumonitis is defined as an inflammatory lung condition due to allergic reactions. When foreign agents like mold or bacteria reach the tiny air sacs (alveoli) in the lungs through the airways (bronchi), they irritate the alveoli and cause sensitive reactions. The flexibility of the air sacs is lost, and it becomes rigid. The gaseous exchange becomes difficult. Hypersensitivity pneumonitis is curable if symptoms are treated earlier.

What Are the Causes of Hypersensitivity Pneumonitis?

The immune system produces inflammatory substances in response to allergic reactions. There is an increased number of white blood cells in the bloodstream. There is a collection of fluid in the alveoli (air sacs) due to the inflammation. The oxygen exchange occurs in tiny air sacs. However, the fluid interrupts the air exchange between the alveoli and blood. The foreign agents that cause hypersensitivity pneumonitis are as follows;

  • Microbes - Microbial organisms like bacteria and fungi or mold irritate the air passages.

  • Chemicals - Harmful chemicals that trigger allergic reactions damage the air sacs (alveoli).

  • Animals and Birds - The fur of animals, the feathers, and the excreta of birds enter the respiratory tract and cause sensitive reactions.

  • Contaminated Food - Hypersensitivity pneumonitis occurs due to the presence of mold in contaminated food like cheese, bread, and barley.

  • Occupational Exposure - The workers involved in wood carving inhale wood dust. This causes allergic reactions in the airways.

  • Home Appliances - Hot tubs, air coolers, and humidifiers also cause hypersensitivity pneumonitis due to the growth of molds. This condition is also known as the humidifier lung.

  • Drugs - Medications like antibiotics, drugs used in chemotherapy for cancer, and cardiac failure have potential side effects like pneumonitis.

  • Radiation Therapy - Radiation therapy is given to patients during bone marrow transplants or to treat lung tumors affected by hypersensitivity pneumonitis later due to high-frequency radiation exposure.

  • Farmers - The mold present in the grains and hay is inhaled by the farmers during harvest. Pesticides are used to prevent insects. The aerosol mist reaches the air sacs through the nasal openings. It results in hypersensitivity pneumonitis.

What Is the Pathophysiology of Hypersensitivity Pneumonitis?

Types III and IV hypersensitivity reactions combine to cause lung parenchymal inflammation in HP. After the first stimulation, the offending antigen or chemical agent first causes a type III hypersensitivity reaction. As a result, serum can show elevated titers of particular immunoglobulin G antibodies in acute HP. A delayed (type IV) hypersensitivity reaction results from continued exposure to the antigen. This triggers the release of chemokines by CD8 cytotoxic T cells, which in turn activate macrophages and cause the formation of granulomas.

What Are the Symptoms Associated With Hypersensitivity Pneumonitis?

The initial symptoms appear six hours after the entry of irritants into the air sacs. The symptoms of hypersensitivity pneumonitis are similar to the normal flu. If the symptoms do not subside in a few days, then consider it chronic pneumonitis. It is a long-term condition. The symptoms are as follows;

  • Fever - There is a high temperature due to foreign body reactions.

  • Chills - The respiratory illness due to the inflammation causes chills.

  • Myalgia and Arthralgia - The weak immune system affects the joints and muscles. The patient has pain in the muscles and joints.

  • Headache - Frequent headache is present in hypersensitivity pneumonitis.

  • Cough - The irritation in the upper respiratory tract causes a dry cough with slight discomfort.

  • Chest Pain - In severe hypersensitivity pneumonitis, both the lungs are involved. It results in chest pain.

  • Weight Loss - The patient feels tired and loses weight due to the loss of appetite.

What Physical Effects Does Hypersensitivity Pneumonitis Cause?

The body overreacts to airborne particles (allergens) when one has hypersensitivity pneumonitis; this usually happens at home or work. The allergens are sufficiently small to cause inflammation even inside the alveoli, the tiny air sacs in the lungs.

The human body produces chemicals to identify the allergen when it is first encountered. The body becomes more sensitive to the allergen and reacts with worsening reactions the more times one is exposed to it. This causes damage to the alveoli and the tiny airways that lead to them over time. This results in symptoms such as coughing and dyspnea.

Who Is Affected by Hypersensitivity Pneumonitis?

Hypersensitivity pneumonitis is more common in people who work in certain industries or are exposed to particular allergens. The likelihood of developing HP increases if individuals:

  • Work on a farm (with vegetables or cattle, for example).

  • Work with animals (veterinarians, people who handle birds or poultry).

  • Prepare and fill with flour or grains.

  • Produce paper and wallboard.

  • Are employed in the metal industry.

  • Are close to feathers or bird droppings.

  • Are in the age range of 50 to 70.

How Is Hypersensitivity Pneumonitis Diagnosed?

The hypersensitivity pneumonitis condition is reversible if the person stays away from the triggering agent. The medical professionals take a few diagnostic tests to confirm the hypersensitivity pneumonitis. They are as follows;

  • Blood Test - The blood test is useful to distinguish hypersensitivity pneumonitis from other respiratory illnesses. The number of eosinophils increases in the blood due to allergic reactions in the lungs.

  • Chest X-rays - This test is painless and covers the areas in the chest. The radiographic image captures both the lungs to identify any abnormality.

  • Computerized Tomography (CT) Scan - The time taken to scan the chest is around 15 minutes. The inflammation causes changes in the lung tissue. This can be assessed using computed tomography. It gives a comprehensive image of the lungs, airways, and air sacs.

  • Pulmonary Function Test - The amount of air exchanged during inspiration and expiration is measured with the help of a special diagnostic device called a spirometer. For this, the patient must fill the lungs during inspiration. The time taken by the person to expire the carbon dioxide is measured using a spirometer. The results are useful for identifying the working capacity of the lungs.

  • Bronchoscopy - The surgeon inserts the flexible tube into the oral cavity to reach the air sacs. The fluid present in the air sacs is collected as a sample for lab examination. A saltwater solution is used to remove the cells from the lungs. This surgical procedure is also known as lavage.

  • Biopsy - Major tissue cannot be removed using bronchoscopy. In that case, surgical removal of the lung tissue is performed. This procedure is known as a biopsy. The sample is used to observe respiratory ailments.

What Are the Treatment Methods for Hypersensitivity Pneumonitis?

  • Corticosteroids - Steroids are used to reduce the inflammatory changes in the lungs and airways. The most commonly used corticosteroid to treat hypersensitivity pneumonitis is Prednisone.

  • Oxygen Therapy - If the patient has shortness of breath due to pneumonitis, artificial oxygen support or hyperbaric oxygen therapy can be given to ease the breathing. The oxygen can be delivered through the mask.

  • Bronchodilators - The hypersensitivity reaction in the airways causes inflammation and narrowing of the bronchi (airways). This results in breathing difficulty. Bronchodilators are prescribed to dilate the air passages for easier air exchange in the lungs.

  • Surgery - If there is severe damage to the lungs due to hypersensitivity pneumonitis, the patient experiences respiratory failure. In that case, the patient needs lung transplant surgery to facilitate the respiratory mechanism.

What Are the Complications of Hypersensitivity Pneumonitis?

Pulmonary fibrosis and eventual respiratory failure are possible outcomes of HP. The existence of honeycombing and fibrosis has been linked to increased mortality. It may also result in pulmonary hypertension.

How Can Hypersensitivity Pneumonitis Be Prevented?

Chronic HP results in irreversible damage. Steer clear of allergens that irritate the lungs to prevent hypersensitivity pneumonitis. There are a few strategies one can try to lower the risk:

  • Wear personal protective equipment (PPE) when working if the profession puts one in danger. Wearing a mask that filters out small particles is part of this.

  • Maintain the cleanliness and functionality of heating and cooling systems, hot tubs, and humidifiers.

  • Steer clear of bedding that contains feathers.

  • Maintain the cleanliness of the pets' living areas, particularly bird cages.

What Is the Prognosis of Hypersensitivity Pneumonitis?

The extent of lung injury determines the prognosis for persistent hypersensitivity pneumonitis. Research indicates that individuals may see a noticeable improvement within the first year of detection if they do not have lung scarring. In the absence of a lung transplant, HP frequently results in death in a matter of years for people with lung scarring.


Severe hypersensitivity pneumonitis causes scarring in the lung tissue. This damages the lungs and is a permanent condition. The scarring makes the lung stiff and slows down respiration. Untreated hypersensitivity pneumonitis leads to pulmonary fibrosis in the future. Many people recover with mild symptoms. Long-term hypersensitivity pneumonitis is a very rare condition. It often occurs in elderly people above the age of 50 years. Avoid working in a field filled with dust particles.

Dr. Kaushal Bhavsar
Dr. Kaushal Bhavsar

Pulmonology (Asthma Doctors)


hypersensitivity pneumonitis
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