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Allergic Reaction

Allergic Reaction Overview

An allergic reaction is the body's way of responding to an "invader." When the body senses a foreign substance, called an antigen, the immune system is triggered. The immune system normally protects the body from harmful agents such as bacteria and toxins. Its overreaction to a harmless substance (an allergen) is called a hypersensitivity reaction, or an allergic reaction.

  • Anything can be an allergen. Dust, pollen, plants, medications (such as ibuprofen, sulfa drugs such as sulfamethoxazole and trimethoprim [Bactrim], codeine, amoxicillin [Amoxil, Amoxil Pediatric Drops, Moxatag, Trimox], cephalexin [Keflex]), foods (common food allergies include shrimp and other shellfish, peanuts), insect bites (such as mosquito bites or bee stings), animal dander, viruses, or bacteria are examples of allergens.
  • Reactions may occur in one spot, such as a small localized skin rash, itchy eyes, face bumps, or all over, as in a whole body rash such as hives (urticaria).
  • A reaction may include one or several symptoms.

Most allergic reactions are minor, such as a rash from poison ivy, mosquito or other bug bites, or sneezing from hay fever. The type of reaction depends on the person's immune system response, which is sometimes unpredictable.

In rare cases, an allergic reaction can be life-threatening (known as anaphylaxis). The Asthma and Allergy Foundation of America (AAFA) estimates that at least one in 50 Americans (1.6%), and as many as one in 20 (5.1%) have had anaphylaxis occur, resulting in an average of 186 to 225 deaths per year.

Allergies are very common. The AAFA states that allergies affect 50 million Americans, are the fifth leading chronic disease in the U.S., and the third leading chronic disease among children under age 18. More than 40 million people have indoor/outdoor allergies as their primary allergy. More than 17 million people visit their doctor for allergies annually, and food allergies account for 50,000 visits to the emergency room annually.

Medically Reviewed by a Doctor on 3/16/2015

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