Merck Manual

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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 blood cells produce antibodies, specifically IgE antibodies, that prepare the immune system for the next encounter with that same allergen. This first exposure to pollen will not produce any outward allergic symptoms, but inside, the IgE antibodies attach themselves to mast cells. Mast cells are cells that can be found in the respiratory system, gastrointestinal tract, and skin.

    During the second and subsequent exposures to pollen, this allergen will combine with the IgE antibody and release chemicals, such as histamine, in the mast cells, thus producing the allergy symptoms of a runny nose, watery eyes, and sneezing.

    There are many types of allergies, such as skin and food allergies, and many different types of allergic reactions, which can range from skin rash to vomiting and diarrhea.