Allergy is a common cause of respiratory diseases, producing hay fever, asthma, and certain forms of pneumonia. Exaggerated immune reactions to environmental substances such as pollens, organic dusts, and chemicals lead to these disorders. When these agents are inhaled, the body overreacts, producing the symptoms of the specific illness.
Asthma, particularly in children, may be triggered by allergic reactions. Allergic pneumonia (hypersensitivity pneumonia) includes such disorders as farmer's lung, an allergy to bacteria in moldy hay, and pigeon breeder's lung, a reaction to proteins in pigeon droppings, as well as many others.