Hypersensitivity pneumonitis: A fibrosing alveolitis produced by inhalation of diverse antigens

Hypersensitivity pneumonitis (HP) is a TH1 lymphocyte –biased fibrosing alveolitis caused by antigens ranging from avian excreta, fungi, thermophilic bacteria, and protozoa to reactive chemicals found in the workplace. Mimicking a viral syndrome, acute exposures to inciting antigens cause abrupt onset of nonproductive cough, dyspnea, and chills with arthralgias or malaise usually from 4 to 8 hours later so that the temporal relationship between antigen exposure and symptoms might be unsuspected. The histology of HP reveals prominent lymphocyte infiltrates that thicken the alveolar septa with poorly formed granulomas or giant cells. (Source: Journal of Allergy and Clinical Immunology)

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