Hypersensitivity pneumonitis

Group of lung diseases typically caused by inhalation of organic antigen to which the individual has been previously sensitized. Disease following inhalation of inorganic antigens and drug ingestion is also reported. Hypersensitivity pneumonitis (HP; previously termed extrinsic allergic alveolitis) is often divided into ‘acute’ and ‘chronic’ forms, based on the time course of presentation. Acute HP often follows a short period of exposure to a high concentration of antigen and is usually reversible. Chronic HP typically follows a period of chronic exposure to a low antigen dose and is less reversible. These two presentations may overlap, and ‘subacute’ forms of the disease are recognized.