Farmer's lung, not to be confused with silo-filler's disease, is a hypersensitivity pneumonitis induced by the inhalation of biologic dusts coming from mouldy hay or other agricultural products. The immune response is most often initiated by exposure to thermophilic actinomycetes, which generates IgG-type antibodies that circulate in the bloodstream. Following a subsequent exposure, IgG antibodies combine with the inhaled allergen and form immune complexes. These complexes are deposited in the lung and generate an inflammatory response typical of a type III hypersensitivity response.
Farmer's lung is an example of extrinsic allergic alveolitis.