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.Allergies can also affect the digestive system, with nausea, vomiting, stomach pain and diarrhea. In most cases, the complaints are due to the combined action of several allergens. Some people may respond to many common environmental allergens, and this inherited predisposition is called atopy.