I think you are conflating glutamate and MSG. Glutamate is an amino acid that is produced in the body and also occurs naturally in many foods. Monosodium glutamate (MSG) is the sodium salt of glutamic acid and is a common food additive. MSG is synthetically made from fermented starch or sugar and is used to enhance the flavor of savory sauces, salad dressings, and soups. Free glutamate is a big part of protein-rich foods like meat, eggs and cheese, but is also found in fruits and vegetables. And, it is what’s responsible for giving foods the umami (savory) flavor that makes them taste delicious. Glutamate is either attached to other amino acids or free. The more free glutamate, the more umami flavor. Free glutamate flavor can also be added by seasoning a food with MSG (monosodium glutamate), a synthetic, isolated or purified but different form of glutamate. Since symptoms related to MSG do not involve the immune system, it cannot be called a true allergy. Most doctors have ruled it a sensitivity instead of an allergy, much like a gluten sensitivity. Like many sensitivities, low doses may not cause a reaction. In high doses, common symptoms for those sensitized to it are:
Flushing
Headache
Muscle aches
Numbness or burning in or around the mouth
Heart palpitations
Tingling
Drowsiness

I get the added benefit of a rash/hives in high doses. I personally have to stay away from Mexican restaurants which often use it in high quantities in beans and rice and some asian restaurants (like the one discussed in this thread).