ABSTRACT

Hypersensitivity pneumonitis (HP), also called extrinsic allergic alveolitis,

is an immunologically induced lung disease caused by repeated inhalation of a variety of causative agents (1-3). Historically, many types of HP that

are related to occupation, such as farmer’s lung, bagassosis, mushroom

worker’s lung, maple bark disease, bird fancier’s disease, and chemical

worker’s lung, were reported between 1930 and 1970. Since 1970, HP

related to the home environment, such as humidifier lung, air conditioner

disease, and summer-type HP, has been reported. At present, it is recog-

nized that HP may present in a number of clinical groups, depending on

factors such as the immunological response of the patient, which is influenced by both genetic and environmental factors, the duration and

amount of exposure to the inhaled antigen, and the nature of the antigen.

Although there are many groups of HP, the clinical and pathologic find-

ings are similar, regardless of the inhaled causative agent or the environ-

mental setting. Clinical presentation is classified as acute, subacute, or

chronic, depending on the amount of inhaled antigen and the exposure

pattern (3). Acute HP presents as cough, dyspnea, fever, rales, leukocyto-

sis, pulmonary infiltrates, and restrictive pulmonary function, usually

beginning four to six hours after intense exposure to the causative agent. Subacute HP presents with a more insidious onset of cough, dyspnea on

exertion, fatigue, anorexia, and weight loss. Chronic HP may progress to

pulmonary fibrosis, pulmonary insufficiency, and death. Typical histo-

pathology includes granulomatous interstitial pneumonitis, with varying

degrees of distal bronchiolitis obliterans. These clinical and pathologic

findings suggest a similar pathogenesis in the development of the pulmon-

ary lesions characteristic of the disease. It is estimated that less than 1% and up to 30% of an exposed population will develop HP, highlighting the role played by the host’s genetic background as well as the environmental

factors and cofactors in development of the disease.