ABSTRACT

Hypersensitivity pneumonitis (HP) is a diffuse parenchymal lung disease produced by the immunological response to an inhaled antigenic agent, usually as a result of intense and sometimes prolonged exposure in the workplace, through a hobby, or in the home. In the United Kingdom the condition is referred to as extrinsic allergic alveolitis (EAA). Al­ though the list of agents and exposures that may cause the disease is long and growing, the pathogenetic mechanisms and clinical features are the same in all instances. The common features appear to be (1) an intense and/or repeated exposure to an inhaled, usually particu­ late antigen small enough to reach the distal lung; (2) a susceptible individual; (3) a T lymphocyte-driven cell-mediated pulmonary immune response that results in transient or persistent lung parenchymal inflammation; and (4) possible fibrosis if the disease persists. The clinical features are often highly suggestive but may be nonspecific and confused with other diseases. Treatment rests on recognition and avoidance of the offending antigen.

HP can present in an acute form characterized by distinct episodes of brief illness or in a chronic form with gradually progressive symptoms. Both patterns respond favorably to antigen removal. In a minority of patients, the chronic form can manifest diffuse interstitial scarring and resemble idiopathic pulmonary fibrosis (IPF). Some authors refer to the chronic progressive pattern as “ subacute” and identify only the end-stage fibrotic disease as “ chronic.” The pattern that develops in a particular patient will depend on the antigen, the nature of the exposure, and possibly on the immune response characteristics of the individual.