Allergic reactions are sensitivities to substances, called allergens, that come into contact with the skin, nose, eyes, respiratory tract, and gastrointestinal tract. They can be inhaled into the lungs, swallowed, or injected.

Allergic reactions are common. The immune response that causes an allergic reaction is similar to the response that causes hay fever. Most reactions happen soon after contact with an allergen.

Many allergic reactions are mild, while others can be severe and life-threatening. They can be confined to a small area of the body, or they may affect the entire body. The most severe form is called anaphylaxis or anaphylactic shock. Allergic reactions occur more often in people who have a family history of allergies.

Substances that don't bother most people (such as venom from bee stings and certain foods, medications, and pollens) can trigger allergic reactions in certain people.

Although first-time exposure may only produce a mild reaction, repeated exposures may lead to more serious reactions. Once a person has had an exposure or an allergic reaction (is sensitized), even a very limited exposure to a very small amount of allergen can trigger a severe reaction.

Most severe allergic reactions occur within seconds or minutes after exposure to the allergen. However, some reactions can occur after several hours, particularly if the allergen causes a reaction after it has been eaten. In very rare cases, reactions develop after 24 hours.

Anaphylaxis is a sudden and severe allergic reaction that occurs within minutes of exposure. Immediate medical attention is needed for this condition. Without treatment, anaphylaxis can get worse very quickly and lead to death within 15 minutes.

Common allergens include:

• Animal dander
• Bee stings or stings from other insects
• Foods, especially nuts, fish, and shellfish
• Insect bites
• Medications
• Plants
• Pollens

Images

Dermatographism - arm

Dermatographism on the back

Allergic reactions

Dermatographism - close-up

Hives (urticaria) on the chest

Dermatographism on the arm

Hives (urticaria) - close-up

Allergic reactions

Hives (urticaria) on the arm

Hives (urticaria) on the trunk

Review Date: 05/02/2010
Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Stuart I. Henochowicz, MD, FACP, Associate Clinical Professor of Medicine, Division of Allergy, Immunology, and Rheumatology, Georgetown University Medical School. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org)