There's a difference between food allergies and food intolerance, although both conditions can elicit the same symptoms. That's why people often mistake intolerance to MSG for an allergy. However, MSG "allergies" are actually intolerances. The difference is that a food allergy is an immune system response; when your body comes in contact with a protein you're allergic to, antibodies called immunoglobulin E are released, and they trigger a slew of chemicals that wage war against the invading allergen. Meanwhile, food intolerance is more likely a digestive system response to something your body can't figure out how to break down. MSG, or monosodium glutamate, is an artificial flavor enhancer that's known to cause intolerance in many people.
The symptoms of MSG intolerance mimic those of an allergy; they can include nausea, vomiting, cramps, diarrhea, gas, heartburn and headaches. The best way to prevent symptoms is to avoid MSG.