Hypersensitivity pneumonitis or extrinsic allergic alveolitis is a rare immune system disorder that affects the lungs. It is an inflammation of the alveoli within the lung caused by hypersensitivity caused by the inhalation of a variety of antigens in the environment, mostly derived from bird proteins, organic dusts, and fungi. Diagnosis is based on an accurate exposure history, clinical presentation, characteristic high-resolution CT findings, specific IgG antibodies to the offending antigen, bronchoalveolar lavage and pathological features. Complete antigen avoidance is the mainstay of treatment.
