Pneumonitis is a general term for inflammation of lung tissue.
Hypersensitivity pneumonitis (also called allergic alveolitis) is an inflammation in the lungs caused by exposure to an allergen, usually organic dust (like fungus spores from moldy hay or bird droppings). Since these allergens are common in certain lines of work (farming, for example), hypersensitivity pneumonitis is usually considered an occupational disease.
When a person inhales such allergens the first time, no symptoms appear. After repeated exposure to the allergen, the immune system of an allergic person becomes sensitized. Once sensitized, the immune system can quickly detect the allergen when it enters the body and produce antibodies called immunoglobulin E (IgE) to fight off the allergen. These antibodies trigger the release of chemical mediators, which cause allergy symptoms, such as hives, rash, teary eyes and runny nose.
Acute hypersensitivity pneumonitis may occur in sensitized individuals four to six hours after exposure to the allergen. The alveoli (tiny air sacs in the lung) become inflamed, and their walls fill up with white blood cells. In some cases, the sacs will fill with fluid. Chronic illness may develop after frequent exposure of high concentrations of the allergen. If the disease is chronic and recurs as a result of continued allergen exposure, parts of the lung may develop fibrous scar tissue (also known as pulmonary fibrosis) and may no longer function normally. At this point, the damage is usually irreversible.
The prevalence of hypersensitivity pneumonitis depends on the region, climate and farming practices. In the United States, it was estimated in 2003 that the disease affects 0.4-7% of the farming population. In the United Kingdom, it was estimated in 2003 there are between 420-3,000 cases of farmer's lung per 100,000 people at risk each year. In France, there are about 4,370 cases per 100,000 at risk each year. In Finland, there are a reported 1,400-1,700 cases per 100,000 people at risk each year.
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Most individuals who have hypersensitivity pneumonitis are exposed to allergens (usually organic dusts) in daily work or personal life. The risk is greater when individuals are exposed frequently to high concentrations of the substances.
However, it is estimated that only about 5-20% of individuals exposed to the known allergens develop the disease.