Allergies are one of the most common chronic diseases. A chronic disease lasts a long time or occurs often. An allergy occurs when the body’s immune system sees a substance as harmful and overreacts to it. The substances that cause allergic reactions are allergens. When someone has allergies, their immune system makes an antibody called immunoglobulin E (IgE). These antibodies respond to allergens. The symptoms that result are an allergic reaction.