So really what you're talking about is glutamate which is a very widely occuring amino acid. It's "non-essential" which means that your body produces it. Almost all fermented foods contain it (including liquid aminos). Cheeses, meats, vegetables, etc contain it. It's very likely that you eat several times as much naturally occurring glutamate in your diet as you would if you used MSG in your cooking.
Basically if someone says they are allergic to MSG, but not allergic to all of the other foods that contain glutamate, is like someone saying they're allergic to table salt, but not allergic to all of the other foods that contain sodium, which is not medically plausible.
The flip side is that very high blood concentration of sodium can cause medical issues, and people worried about glutamate argue that a similar effect is going on when you eat MSG, but research so far indicates that at normal dietary levels, you're fine to sprinkle a little of either molecule on your food.
