Hypersensitivity pneumonitis is an inflammation of the lung (usually of the very small airways) caused by the body's immune reaction to small air-borne particles. These particles can be bacteria, mold, fungi, or even inorganic. Hypersensitivity pneumonitis (extrinsic allergic alveolitis) includes numerous examples that are caused by specific antigens. Farmer's lung,
associated with repeated inhalation of dusts from hay containing thermophilic actinomycetes, is the prototype.
Hypersensitivity pneumonitis is usually an occupational disease in which exposure to organic dusts, fungus, or molds leads to acute and over time, chroniclung disease. Exposure may also occur in the home, from fungus (molds) present in humidifiers, heating systems, and air conditioners. Some people (for example, bird owners) may have hobbies that can lead to exposure. Acute illness may occur 4 to 6 hours after the exposure, once the person has left the area where the allergen is present. Chronic illness with changes seen on chest X-ray may develop with continued exposures. The chronic form of this disease may lead to pulmonary fibrosis (a scarring of the lung tissue that is often not reversible). Hypersensitivity pneumonitis is thought to be immunologically mediated, although pathogenesis is not completely clear. Precipitating antibodies to the causative antigen are usually demonstrated, suggesting a type III reaction, although vasculitis is not common. Type IV hypersensitivity is suggested by the granulomatous primary tissue reaction and findings in animal models.
Hypersensitivity pneumonitis can range from acute to chronic. Acute hypersensitivity pneumonitis tends to occur 4-12 hours after exposure (usually heavy exposure) to the particles. Symptoms include fever, chills, coughing, shortness of breath, body aches, etc. Chest x-rays may show diffuse small nodules. Typically, the symptoms will subside hours to days after exposure (provided there are no repeated exposures). Chest x-ray abnormalities will also disappear. Chronic hypersensitivity pneumonitis causes lung scarring (fibrosis) and symptoms of shortness of breath and cough. Chronic disease is believed to occur after prolonged low grade exposure to the offending particles.
Treatment of hypersensitivity pneumonitis seeks to identify the offending allergen and avoid further exposure to it. A change of occupations may be indicated in cases where future worksite exposure would be unavoidable. In chronic forms of the disease, treatment with glucocorticoids (a type of steroid drugs) can be tried because this may decrease inflammation.