Inflammation of the bronchial tree, interstitial tissue, and alveoli of the lung caused by a hypersensitivity reaction to an inhaled antigen. With repeated exposure, large numbers of macrophages form granulomas, which damage and scar lung tissue. The inhaled allergens that most often trigger allergic alveolitis are molds and other fungi, vegetables, mushrooms and mushroom compost, flour, tree bark, detergents, and contaminated humidifiers. In the acute stage, patients may present with cough, fever, chills, malaise, and shortness of breath. In the subacute and chronic forms, the onset of symptoms is gradual and prolonged. Farmer’s lung and bagassosis are two common names for forms of allergic alveolitis.