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First described in 1874, hypersensitivity pneumonitis (HP), or extrinsic allergic alveolitis, is an inflammatory disorder of the lung involving alveolar walls and terminal airways that is induced by repeated inhalation of a variety of organic agents in a susceptible host. The expression of HP depends on factors related to the host susceptibility and the inciting agent. The frequency of HP varies with the environmental exposure and the specific antigen involved, which often depends on season, geographic location, or presence of certain industries.




Agents implicated as causes of HP are diverse and include those listed in Table 255-1. The common name of each disease often reflects the occupational or avocational risk associated with that disease. In the United States, the most common types of HP are farmer's lung, bird fancier's lung, and chemical worker's lung. In farmer's lung, inhalation of proteins, such as thermophilic bacteria and fungal spores that are present in moldy bedding and feed, are most commonly responsible for the development of HP. These antigens are probably also responsible for the etiology of mushroom worker's disease (moldy composted growth medium), bagassosis (moldy sugar cane), and water-related exposure (molds in air conditioners or humidifiers). Hot tub lung refers to a hypersensitivity reaction to Mycobacterium avium complex, which is present in hot tubs or whirlpools and is differentiated from actual infection. Bird fancier's lung (and the related disorders of duck fever, turkey handler's lung, and dove pillow's lung) is a response to inhalation of proteins from feathers and droppings. Chemical worker's lung is an example of how simple chemicals, such as isocyanates, may also cause immune-mediated diseases. Interestingly, cigarette smoking has been associated with decreased incidence of HP; however, smoking may lead to a more progressive or severe course of HP once the disease is present.

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Table 255-1 Selected Examples of Hypersensitivity Pneumonitis (HP) 

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