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A to Z: Allergy

Also called: Chronic Allergies, Seasonal Allergies, Hay Fever, Food Allergies, Insect Allergies

An allergy is the immune system's overreaction to certain plants, animals, foods, insect bites, or other things. The immune system protects a person from diseases by fighting germs like bacteria and viruses.

But with allergies, the immune system overreacts and tries to "fight" allergens, which are ordinary things that cause an allergic reaction (like grass, pollen, or certain foods). The tendency to develop allergies runs in families.


When trying to protect the body, an allergic person's immune system makes antibodies called immunoglobulin E (IgE). These antibodies then cause allergy cells in the body (called mast cells and basophils) to release chemicals, including histamine, into the bloodstream. These chemicals help defend against the allergen "invader."

Releasing these chemicals causes allergic reactions as the body tries to get rid of the invading allergen. This causes the sneezing, itching, watery eyes, and other reactions that occur with allergies.

When the immune system reacts to an allergen and causes symptoms, the person is allergic to it. Being exposed to that same allergen will trigger an allergic response again. This means that every time the person eats that particular food or is exposed to that particular allergen, he or she will have an allergic reaction.

Doctor can determine the cause of an allergy with different tests. The most common allergy tests are skin tests and blood tests. These tests can be done in infants, but they're more reliable in kids over 2 years old.

Key Point

An allergy is a chronic (long-lasting) condition. In treating allergies, doctors focus on relieving the symptoms. The ideal way to live with allergies is to reduce or avoid being around allergens.

All A to Z dictionary entries are regularly reviewed by KidsHealth medical experts.

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