lung and airway disorders
Hypersensitivity Pneumonitis
Causes of Hypersensitivity Pneumonitis
Many substances can cause hypersensitivity reactions in the lungs. Organic dusts that contain microorganisms or proteins, and chemicals, such as isocyanates, may cause hypersensitivity pneumonitis. Farmer's lung, which results from repeated inhalation of heat-loving (thermophilic) bacteria in moldy hay, is a well-known example of hypersensitivity pneumonitis. Bird fancier's lung is another example. It occurs when dust from the droppings or feathers of birds (either on living birds or in pillows and comforters) is inhaled. Only a small number of people who inhale certain common dusts develop hypersensitivity reactions. Generally, a person must be exposed repeatedly over time before sensitivity and resultant disease develop. Lung damage appears to result from damage done by lymphocytes, a type of White blood cells . Initial exposures to the dusts sensitize lymphocytes. Some lymphocytes then help to produce antibodies that play a role in tissue damage. Other lymphocytes participate directly in inflammation after subsequent antigen exposure. Recurrent exposure to the antigen results in a chronic inflammatory response, which is manifested by a buildup of white blood cells in the walls of the alveoli and small airways. This buildup leads progressively to symptoms and disease. 