What Is an Allergy? Allergies are an abnormal response of the
immune system. People who have allergies have an immune system
that reacts to a usually harmless substance in the environment.
This substance (pollen, mold, and animal dander, for example) is
called an allergen. Allergies are a very common problem,
affecting at least two out of every 10 Americans. What Happens
During an Allergic Reaction? First, a person is exposed to an
allergen by inhaling it, swallowing it, or getting it on or
under their skin. After a person is exposed to the allergen, a
series of events create the allergic reaction: - The body starts
to produce a specific type of antibody, called IgE, to bind the
allergen. - The antibodies attach to a form of blood cell called
a mast cell. Mast cells can be found in the airways, in the
intestines, and elsewhere. The presence of mast cells in the
airways and GI tract makes these areas more susceptible to
allergen exposure. - The allergens bind to the IgE, which is
attached to the mast cell. Thi