An allergy is a chronic condition involving an abnormal reaction to an ordinarily harmless substance called an allergen*. If you have an allergy, your immune system views the allergen as an invader and a chain reaction is initiated. White blood cells of the immune system produce IgE antibodies. These antibodies attach themselves to special cells called mast cells, causing a release of potent chemicals such as histamine. These chemicals cause symptoms such as: itching (in the nose, roof of the mouth, throat, eyes), sneezing, stuffy nose (congestion), runny nose, tearing eyes, dark circles under the eyes.