The allergic reaction is misguided in that these foreign substances are usually harmless.The allergic reaction occurs when a person's immune system becomes hypersensitive to certain substances, such as foods, pollen, medications, or bee venom. A substance that causes an allergic reaction is called an allergen. Many allergens are everyday substances that are harmless to most people.The symptoms of an allergic reaction include:sneezing and an itchy, runny or blocked nose, red and itchy  watering eyes, wheezing chest tightness, shortness of breath and a cough, a raised, itchy, red rash, swollen lips, tongue, eyes or face.