allergy

Special sensitivity of the body that makes it react with an exaggerated response of the natural immune defence mechanism to the introduction of an otherwise harmless foreign substance (allergen).

Allergic symptoms are caused by the release of active substances from cells following the reaction of the body's immune defence mechanism to the allergen. These substances are histamine, serotonin, a slow-reacting substance, and bradykinin. The liberation of histamine has two important physiological actions. First, it causes the small blood vessels to leak so that fluid is poured out into the tissues (the effect may be so pronounced that the blood pressure may fall and unconsciousness ensue; this is called anaphylactic shock). Secondly, it causes spasm of smooth muscle, for example, of the lungs.

Hay fever in summer is caused by an allergy to one or more kinds of pollen. Many asthmatics are allergic to certain kinds of dust or to micro-organisms in animal fur or feathers. There are many different kinds of allergies, producing a range of effects. Some people experience skin reactions, such as dermatitis, from drugs, cosmetics, or household cleaners; some suffer gastric upsets due to allergens in food. A severe allergic reaction may produce profound shock. Various drugs can be used to counteract allergic symptoms, including antihistamines and steroids.