Hypersensitivity pneumonitis

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By Medifit Education



Hypersensitivity pneumonitis 2


Hypersensitivity pneumonitis is a disease of the lungs in which your lungs become inflamed as an allergic reaction to inhaled dust, fungus, molds or chemicals.



Repeatedly breathing in foreign substances can cause hypersensitivity pneumonitis (HP). Examples of these substances include molds, dusts, and chemicals. (Mold often is the cause of HP.) These substances also are known as antigens.

Over time, your lungs can become sensitive to antigens. If this happens, your lungs will become inflamed, which can lead to symptoms and may even cause long-term lung damage.

Antigens may be found in the home, workplace, or in other settings. Antigens can come from many sources, such as:

  • Bird droppings
  • Humidifiers, heating systems, and hot tubs
  • Liquid chemicals used in the landscaping and florist industries
  • Moldy hay, straw, and grain
  • Chemicals released during the production of plastics and electronics, and chemicals released during painting
  • Mold released during lumber milling, construction, and wood stripping



Hypersensitivity pneumonitis is a spectrum of immune-mediated disorders characterized by diffuse inflammation of interstitial lung, terminal bronchioli, and alveoli. Inflammation is caused by prolonged or frequent exposure of inhaled antigens generally less than 5 µm in size. Although offending antigens are ubiquitous, the incidence of disease is small. Thus, disease is thought to occur via a two-hit mechanism in which genetic predisposition or environmental exposure (first hit) contribute to progression to HP following antigen exposure (second hit). Type III and Type IV hypersensitivity reactions are implicated.

Within the organic dusts responsible for HP, including dusts from dairy and grain products, animal dander and proteins, wood bark, and water reservoir vaporizers (eg, hot tubs, air conditioners), are antigens consisting of spores from thermophilicactinomycetes (eg, Saccharopolysporarectivirgula) or fungi (eg, Aspergillusspecies). High- and low-molecular-weight animal proteins, often from bird serum, feces, or feathers are also common antigens. Direct handling of birds may not be necessary as HP has also been described following the use of feather down items. Finally, low-molecular weight molecules and inorganic materials (eg, isocyanates, zinc, and nickel) may serve as haptens, forming antigenic complexes when combined with host proteins.

Most individuals do not react to these antigens, likely secondary to development of immune tolerance. These individuals may develop a mild lymphocytic alveolitis, but generally remain asymptomatic. Tolerance is thought to be mediated by regulatory T cells, which suppress the Th1 and Th2 cell immune responses.

In patients who go on to develop symptoms, HP is classified as acute, subacute (intermittent), or chronic progressive. The mechanisms of disease are incompletely understood. Acute HP is thought to occur primarily via type III hypersensitivity reaction. Most patients show evidence of specific antibodies in their serum and bronchoalveolar lavage studies may demonstrate high levels of proinflammatorychemokines. This is further supported by the finding of complement and immunoglobulin deposition in vessel walls on immunofluorescence.

Subacute and chronic forms of HP are thought to transition more towards type IV, T-cell mediated, hypersensitivity reactions. Antigen presenting cells (ie, dendritic cells and alveolar macrophages) present antigens to CD4+ Th1 and Th17 cells. This triggers an inflammatory cascade with release of many factors including IFN-γ, TNF, IL-17, IL-22. The milieu of cytokines and chemokines ultimately results in sustained infiltration of mononuclear cells, macrophages, and fibroblasts. The apoptosis of lung tissue lymphocytes is inhibited by IL-17 resulting in the high prevalence of lymphocytes in the lung. This in turn results in the pattern of noncaseatinggranulomas, bronchiolitis seen on pathology. In chronic stages, a CD4+ Th2 cytokine pattern dominates. This correlates with fibrotic progression in late disease.

Histologically, chronic HP is characterized by interstitial inflammation and alveolar destruction (honeycombing). Cholesterol clefts or asteroid bodies are present within or outside noncaseating granulomas. Areas of cellular interstitial pneumonia with giant cells or granulomas surrounding bronchioles may help distinguish chronic HP from usual interstitial pneumonia (UIP) or fibrotic nonspecific interstitial pneumonia (NSIP). Centrilobular fibrosis, peribronchiolar and bridging fibrosis are also important hallmarks.

Features often associated with poorer prognosis include:

  • Predominantly peripheral fibrosis in a patchy pattern with architectural distortion and fibroblast foci similar to UIP
  • Homogeneous linear fibrosis similar to fibrotic NSIP
  • Irregular predominantly peribronchiolar fibrosis

Hypersensitivity pneumonitis 1


Signs and symptoms of hypersensitivity pneumonitis (HP) depend on whether the disease is acute (short-term) or chronic (ongoing).


With acute HP, symptoms usually occur within 2–9 hours of exposure to an antigen you’re sensitive to. (An antigen is a substance that your body reacts against, such as molds, dusts, and chemicals.)

Acute HP can cause chills, body aches, coughing, and chest tightness. After hours or days of no contact with the antigen, symptoms usually go away.


If acute HP isn’t found and treated early, chronic HP may develop. With chronic HP, symptoms occur slowly, over months. Chronic HP can cause a worsening cough, shortness of breath with physical activity, fatigue (tiredness), and weight loss.

Some symptoms may continue and/or worsen, even after avoiding the antigen. Chronic HP can cause long-term lung damage, such as pulmonary fibrosis. This is a condition in which tissue deep in your lungs becomes scarred over time.

Clubbing also may occur if HP is severe. Clubbing is the widening and rounding of the tips of the fingers or toes. A low level of oxygen in the blood causes this condition.



Many people with episodes of hypersensitivity pneumonitis are probably unrecognized and undiagnosed. Some cases believed to be viral pneumonias may actually be hypersensitivity pneumonitis. The patient’s history of repeated episodes of typical symptoms, hours after exposure to certain environments are important in establishing the diagnosis.

Most patients with this disorder have symptoms of shortness of breath and/or cough. Quite often a chest X-ray may show a variety of abnormalities, but primarily increased lung markings. A specialist in lung disease will primarily depend on a history of exposure either by occupation or hobby. With the abnormal chest X-ray, a high resolution cat scan of the chest is often reviewed. This study shows a detailed image of the appearance of the lung tissue. Lung function tests that examine the lung volumes and the ability for gases to move through the lungs (diffusing capacity) are then performed. Not only are the CT scan and pulmonary function tests useful in diagnosis but they are also useful in following response to therapy.

Blood antibody tests and skin tests against certain offending molds, bacteria, or particles are available, but their results are usually inconclusive. Other more recent tests such as the serum KL-6 (Krebs von den Lungen-6; this test may act as a marker for activity seen in diffuse lung disease) may be used. Infectious causes as well as collagen vascular diseases and cancer must be excluded as a cause of the symptoms and signs, and this may involve a biopsy. Unfortunately the small biopsies obtained with a bronchoscope or a needle are usually inadequate. It is often necessary to perform Video Assisted Thoracoscopic Surgery (a.k.a. VATS), which is an open lung biopsy technique performed by thoracic surgeons under general anesthesia. This procedure allows for a larger sample size and more accurate diagnosis.



The best way to treat hypersensitivity pneumonitis (HP) is to avoid the antigen that caused it. (An antigen is a substance that your body reacts against, such as molds, dusts, and chemicals.)

In acute (short-term) HP, symptoms usually go away once you’re no longer in contact with the antigen. In chronic (ongoing) HP, you may need medicines to relieve your symptoms.

People who have chronic HP may develop pulmonary fibrosis. This is a condition in which tissue deep in your lungs becomes scarred over time. People who have this condition may need further treatment, such as oxygen therapy and pulmonary rehabilitation(rehab).


Once the antigen that caused the HP and its source are found, you can take steps to avoid it. If HP is caught early, avoiding the antigen may be the only treatment you need.

Avoiding an antigen may be easier at home than at work. For example, if your pet bird, moldy carpet, or hot tub is the source of the antigen, you can remove it from your home. If your heating system is the source of the antigen, you can have your system properly serviced.

However, if the antigen is at work, you may need to talk with your supervisor about your condition and ways to protect yourself. For example, masks or personal respirators may help protect you from antigens in the air. (A personal respirator is a device that helps filter the air you breathe in.)

Some people who have HP may need to move to a different home or change jobs to avoid antigens. After hurricanes, for example, some people have to move from their homes to avoid molds that could harm their lungs. However, moving and changing jobs sometimes isn’t possible.


If you have chronic HP, your doctor may prescribe medicines called corticosteroids. These medicines reduce lung inflammation. Prednisone is an example of a corticosteroid.

Long-term use of prednisone, especially at high doses, can cause serious side effects. Thus, if your doctor prescribes this medicine, he or she may reduce the dose over time.

Examples of side effects from corticosteroids are increased risk of infections, high blood pressure, high blood sugar, and osteoporosis (thinning of the skin and bones).

People who develop pulmonary fibrosis may need medicines, oxygen therapy, and/or pulmonary rehab. Pulmonary fibrosis is a condition in which tissue deep in your lungs becomes scarred over time.

If you smoke, try to quit. Smoking can make HP symptoms worse and lead to other lung diseases. Talk with your doctor about programs and products that can help you quit. Also, try to avoid secondhand smoke.

By Medifit Education