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An allergy occurs when the body reacts to substances it can't tolerate. The substances, like tree pollen or dust mites, are called environmental antigens, or allergens, and are normally harmless. But the immune system of a person with allergies views the allergens as harmful.

If a person is allergic to pollen, for example, and is exposed to this allergen, here's what happens during the body's immune response:

Following the body's first exposure to the allergen, the white ... more