There are two types of changes. The first is beneficial in that our body builds up resistance to the foreign substance and is the basis for most immunizations. The second type of response is generally not beneficial. It's known as a hypersensitivity response and occurs when the body's immune system reacts to the foreign substance when the response is not necessary.
Food allergies occur at all ages and almost any food can cause an allergic response.
Food allergy is not the only cause of digestive upsets but is blamed for much that it doesn't cause. Food allergy may produce the symptoms of swelling of the mouth and lips, hives, skin rashes, vomiting, diarrhea, asthma, runny nose and other problems. The best approach to determine a food allergy is detecting symptoms that occur after eating a certain food. For example, if your lips swell after eating shrimp, you are probably allergic to shrimp. Asthma is a severe allergic disorder.