Really careful elimination diet:

Eliminate all common food allergens - which i got from an allergist's RAST testing prescription - and foods in the same genus. Also, eliminate foods in the same genus as any food that you normally eat more than once a week. Thus, you eat Really Weird Food for a week!

So, eliminate:

Corn. More difficult than it seems. Lots of toothpaste and mouthwashes have sorbitol, lots of medications have corn starch or corn syrup. Tom's of Maine has corn-free toothpaste. At least, they say it's corn-free. It has xylitol made from birch and beech trees, which I get sick from. Corn might be introduced somewhere in the process of making the xylitol. List of corn allergens. A lot of processed food has corn in it.

Eggs
Dairy products
Nuts, including peanuts
Legumes: Soy - soy is used in a lot of processed foods also; peas, beans, carob
Wheat, rye, barley, oatmeal
beef
poultry
chocolate
citrus fruit
rice
tomatoes
apples
carrots
garlic, leek, onion, shallot, chives
mustard, bok choy, turnip, rutabaga, cabbage, cauliflower, broccoli, brussels sprouts
pork
white potatoes, eggplant
Shellfish
Fish
Strawberries
Yeast containing food - mushrooms, fermented foods
buckwheat
celery
Melons except watermelon, cuke, gherkin

Any food you normally eat more than once/week, and foods in the same genus. A list of plant relationships. This includes whatever you put in your mouth! Vitamins, medications, toothpaste, spices. Try to quit all supplements for the elimination diet. If you take medications that you can't quit, you can maybe get them made without fillers by a compounding pharmacy.