Definition:

Extrinsic allergic alveolitis (EAA), also known as hypersensitivity pneumonitis, is caused by repeated inhalation of mainly organic antigens by sensitized subjects. This induces a hypersensitivity response in the distal bronchioles and alveoli and subjects may present clinically with a variety of symptoms. EAA is caused by a wide variety of antigens including bacteria, organic materials, fungal spores, and chemicals. Of the various EAA syndromes, Farmer's lung and Bird fancier's disease are the commonest forms that have been studied. EAA is a complex dynamic clinical syndrome where clinical expression depends on the frequency and amount of contact with antigen and host susceptibility. The manifestations and symptoms can be complicated by self-regulatory measures performed by those affected individuals to reduce antigen exposure. There are many individuals with a positive antibody reaction who remain asymptomatic. EAA traditionally has been described as occurring in a state of acute, subacute, and chronic form.