I remembered having read something recently about this, but could not remember where. I found the following info online:
"Food allergies usually are due to the food itself, not any additives. True allergies are caused by stimulation of the immune system; reactions to additives probably occur independent of the immune system and may be due to chemical intolerance. It is difficult to verify that food additives are causing symptoms because no specific tests exist for additives. In some people, monosodium glutamate (MSG) may cause sudden, severe headache, but it is not yet known whether it causes allergies. Sulfites, sometimes added as a preservative to salad bar foods, have been documented to cause asthmatic reactions, but not as a true allergen. "
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http://www.ahsc.arizona.edu/opa/health/foodalrg.htm