The term hypersensitivity pneumonitis or extrinsic allergic alveolitis (EAA) is defined as a group of diseases characterized by a response of the lung to specific antigen contained in a wide variety of organic dusts on such fine particles size that they can penetrate into the most distal lung parenchyma. It is characterized by diffuse inflammation of lung parenchyma and airways in sensitized individuals. The main immunological reaction involved in EAA is Type III arthus reaction. Farmer’s lung, the classic example of hypersensitivity pneumonitis, is an allergic disease usually caused by breathing in the dust from moldy hay that can be prevented by taking precautions.