Exaggerated reaction by the body to foreign substances that are harmless to most people.
Those substances, called allergens or antigen s, may include pollen s, drug s, dusts, foods, and other items. Immediate allergic reactions result from genetic predisposition or sensitization by previous exposure. Blood vessels dilate and bronchial air passages constrict. A severe reaction ( anaphylaxis ) can obstruct breathing and may be fatal. Delayed allergic responses (e.g., contact dermatitis ) appear 12 hours or more after exposure. Avoiding allergens and taking antihistamine s can prevent or treat allergies. When avoidance is not feasible and antihistamines do not relieve symptoms, desensitization can be attempted.