Allergies can have an effect on many parts of the body: the eyes, nose, throat, lungs, skin, or gastrointestinal tract. Illustration courtesy of the National Cancer Institute/Terese Winslow, artist

Exposure to what is normally a harmless substance, such as pollen, causes the immune system to react as if the substance is harmful. Substances that cause allergies are called allergens.

An allergy is an overreaction of the immune system to a substance that's harmless to most people, such as pollen. But in someone with an allergy, the body's immune system treats the substance (called an allergen) as an invader and reacts inappropriately, resulting in symptoms that can be anywhere from mildly uncomfortable to potentially life-threatening, in some cases.

In an attempt to protect the body, the immune system of the allergic person produces antibodies called immunoglobulin E (IgE). Those antibodies then cause mast cells (allergy cells in the body) to release chemicals, including histamine, into the bloodstream to defend against the allergen "invader."

It's the release of these chemicals that causes allergic reactions, affecting a person's eyes, nose, throat, lungs, skin, or gastrointestinal tract as the body attempts to rid itself of the invading allergen. Future exposure to that same allergen (things like nuts or pollen that you can be allergic to) will trigger this allergic response again. This means every time the person eats that particular food or is exposed to that particular allergen, he or she will have an allergic reaction.

Allergy Overview from the Asthma and Allergy Foundation of America This is a brief handout with simple suggestions to prevent allergies at home, work school, outside and during travel.

Is it the flu, a cold, or an allergy? from Bay Area Medical Information (BAMI.us)

All about Allergies from the Nemours Foundation This is a more detailed handout about the most common allergies, their symptoms, treatment, and prevention.

Allergy Shots from the Nemours Foundation