Hypersensitivity pneumonitis (HP), also known as extrinsic allergic alveolitis, encompasses a group of pulmonary disorders characterized by immune response to a variety of antigens derived from medications, chemicals, microorganisms, and plant and animal proteins. The first cases of HP were described in occupational or hobbyist settings, but HP secondary to home exposures is becoming more common. The clinical presentation has a wide range, from acute HP, which often presents as an acute febrile illness, to chronic HP, with insidious progression of dyspnea and systemic complaints. The development of HP requires TH1 cell, cytotoxic lymphocyte, and IgG immune responses. TH2 cells and IgE are not involved. The diagnosis of HP must be made clinically with consistent history and known appropriate antigen exposure. Supportive evidence can be gathered from serologic tests (including serum precipitans), radiographic evaluation, bronchoscopy with BAL, biopsy, and natural or direct antigen inhalation challenges. Although systemic corticosteroids may improve acute exacerbations, removal from the inciting antigen is paramount in the appropriate treatment of HP. Unfortunately, irreversible pulmonary fibrosis can develop in long-standing HP leading to significant morbidity and mortality, highlighting the need for early detection and antigen avoidance.
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