Diagnosis: Extrinsic allergic alveolitis (chronic hypersensitivity pneumonitis)
• Silo-filler’s disease will not have the histologicappearance shown. It is the result of a toxic chemical exposure to nitrogen dioxide; this can cause suffocation or acute respiratory distress syndrome (ARDS) and is not immunologically mediated. All of the other answer choices are a cause of chronic hypersensitivity pneumonitis (HP).
• Extrinsic allergic alveolitis (EAA) (chronic HP) is a form of type III/type IV hypersensitivity mediated by IgG and cell-mediated immunity. IgG is the main immunoglobulin of the distal lung. Chronic HP refers to an alveolitis; therefore, the response is a combination of IgG-mediated inflammation with lymphocytes and plasma cells and cell-mediated reaction involving histiocytes.
• IgE-mediated hypersensitivity can also occur in the lung with eosinophilia; this pattern of hypersensitivity is seen in larger airways in processes such as allergic bronchopulmonary aspergillosis.
• As noted previously, chronic HP and EAA are synonyms; as an alveolitis the hypersensitivity is an IgG-mediated response. The spectrum of organic antigens that can cause EAA/chronic HP is wide, although birds, molds, and bacteria are frequent culprits. More recently, cases of hypersensitivity to nontuberculous mycobacteria have been reported in sources of contaminated water (“hot tub lung”).
• Chronic HP without fibrosis responds to steroid therapy and avoidance of the offending antigen if it can be identified. Unfortunately, some cases can lead to severe lung fibrosis and such cases do not have a favorable response. In some instances, this leads to end-stage lung fibrosis mimicking UPI.
• Certain medications such as methotrexate, can mimic HP. Because this is not an inhaled antigen, such cases do not have the prominent bronchiolitis of EAA/chronic HP.
