Hypersensitivity pneumonitis (extrinsic allergic alveolitis)
Hypersensitivity pneumonitis (HP), also known as extrinsic allergic alveolitis (EAA), represents a group of lung diseases caused by repeated inhalation of a specific antigen or antigens in susceptible individuals. The lung is the site of a variety of complex inflammatory reactions in response to endogenous or exogenous stimuli. In HP the inflammation represents a tissue reaction against various environmental agents that vary from thermophilic bacteria, mammalian and avian proteins, and fungi, to a large number of organic particles and, rarely, low molecular weight compounds. Diffuse inflammation of the small airways and the peripheral gas-exchanging portion of the lung are characteristic of HP.1 Although the terms hypersensitivity and allergic might suggest an atopic disorder, HP is not associated with eosinophilia or excessive production of immunoglobulin E (IgE). Hypersensitivity pneumonitis is characterized by a bronchioloalveolitis sustained by CD8+ cytotoxic T lymphocytes, interstitial granulomas, and, over time, the development of interstitial fibrosis. Hypersensitivity pneumonitis has a complex and poorly understood pathogenesis, and although it involves predominantly cell-mediated mechanisms, the production of specific antibodies directed against the offending antigens2 and nonimmunologic factors may play a role. © 2008 Springer New York.
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