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It is a malfunction of the body defense system (immune system), where substances are targeted as harmful when they are not really a threat to the body.
Every substance able to elicit an allergic reaction is known as an allergen. After the body gets in touch with the allergen, a defense response is started and it is the responsible of the characteristic symptoms of the allergic disease.
The immune system produces in excess a special kind of antibody known as Immunoglobulin E (or “IgE” in short). Every human body produce IgE normally, but the allergic patient over produces it and address it against the allergen(s)