Allergies are your body's reaction to a substance it views as a harmful “invader.” For example, coming into contact with what is normally a harmless substance, such as pollen, might cause the immune system (the body's defense system) to react. Substances that cause these reactions are called allergens. What is an allergic reaction? An “allergic reaction ” is way the body responds to the allergen. A chain of events occur that result in an allergic reaction. The first time an allergy-prone person is exposed to a specific allergen (such as pollen), the body responds by producing allergic (IgE) antibodies. The job of these antibodies is to find molecules of the offending substance in the bloodstream and tissues and to usher them to the body's mast cells (a type of white blood cell) for destruction. As the mast cells destroy the allergens, a chemical called histamine is released into the bloodstream. A large amount of histamine swells body tissues (inflammation), causes