Font Size
A
A
A
1
...

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 like sulfamethoxazole and trimethoprim [Bactrim], codeine, amoxicillin [Amoxil, Amoxil Pediatric Drops, Moxatag, Trimox]), foods (common food allergies include shrimp and other shellfish, peanuts), insect bites (such as from mosquitos or bees), 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). Each year in the United States, it is estimated that there are 150,000 cases of anaphylaxis. Up to 500-1,000 fatalities due to anaphylaxis occur each year.

Allergies are very common. The Asthma and Allergy Foundation (AAFA) states allergies affect 50 million Americans, is the fifth leading chronic disease in the U.S., and the third leading among children. 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 3/28/2014

Must Read Articles Related to Allergic Reaction

Allergy Shots
Allergy Shots Some allergens are simply impossible to avoid. You cannot live a normal life and completely avoid pollen, dust mites, mold spores, and certain other common trig...learn more >>
Allergy: Insect Sting
Allergy: Insect Sting Several insects belonging to the class Hymenoptera are capable of injecting venom into humans and animals. These insects include honeybees, bumble bees, hornets...learn more >>
Poison Ivy, Oak, and Sumac
Allergy: Poison Ivy, Oak, and Sumac More than half the people in the United States are sensitive to poison ivy, poison oak, and poison sumac. If you are sensitive, you can develop an itchy, bliste...learn more >>

Patient Comments & Reviews

The eMedicineHealth doctors ask about Allergic Reaction:

Allergic Reaction - Symptoms

What were the symptoms and signs of your allergic reaction?

Allergic Reaction - Treatment

What was the treatment for your allergic reaction?

Allergic Reaction - Experience

Please describe your experience with allergic reaction.

Allergic Reaction - Home Remedies

What home remedies have been effective for your allergic reaction?




Read What Your Physician is Reading on Medscape

Hypersensitivity Reactions, Immediate »

The immune system is an integral part of human protection against disease, but the normally protective immune mechanisms can sometimes cause detrimental reactions in the host.

Read More on Medscape Reference »


Medical Dictionary