Hypersensitivity pneumonitis involves inhalation of an antigen. This leads to an exaggerated immune response and a following inflammation of the alveoli within the lungs. The main feature of chronic hypersensitivity pneumonitis on lung biopsies is expansion of the interstitium by lymphocytes accompanied by an occasional multinucleated giant cell or loose granuloma. After exposure to the provoking antigen, following symptoms can be seen: fever, chills, malaise, cough, hemoptysis, chest tightness, dyspnea, rash, swelling and headache and can be completely reversible, based on the duration of the illness, categorized as acute (HP:0011009), subacute (HP:0011011), and chronic (HP:0011010).
