Food reactions are common, but most are caused by a food intolerance rather than a food allergy. A food intolerance can cause some of the same signs and symptoms as a food allergy, so people often confuse the two.
A true food allergy causes an immune system reaction that affects numerous organs in the body. It can cause a range of symptoms. In some cases, an allergic food reaction can be severe or life-threatening. In contrast, food intolerance symptoms are generally less serious
If you have a food allergy, even a tiny amount of the offending food can cause an immediate, severe reaction. Digestive signs and symptoms may include nausea, vomiting, cramping, and diarrhea. Other signs and symptoms can include a tingling mouth, hives, and swelling of the lips, face, tongue, and throat. A life-threatening allergic reaction known as anaphylaxis can cause breathing trouble and dangerously low blood pressure. If you have a food allergy, you’ll need to avoid the offending food entirely.
Food intolerance symptoms generally come on gradually and don’t involve an immune system reaction.
Source: Mayo Clinic (James T C Li, M.D., Ph.D.) – http://www.mayoclinic.com/health/food-allergy/AN01109