The most common food allergies are milk, egg, peanut, tree nut, fish, shellfish, wheat and soy. Milk is the most common, yet most people recognize peanuts as a major issue. My son Bradley is allergic to 7 of these. He's anaphylactic to dairy, eggs, peanuts and tree nuts. We carry an Epi-pen and Benadryl wherever we go.
If you suspect your child of having a food allergy, I highly recommend the skin test. Within 15 minutes we were able to find out about his sensitivities. Although not 100% accurate, its a great guide to start with. The issue we had is that some of his reactions are delayed. For instance with tomatoes, a reaction would show up 4 days later. Keep track of what you feed your child with a food journal. Every monday, introduce one new food, and wait several days...see if anything happens within the week. You may need to retry foods in order to see a pattern. We gave Bradley tomatoes for 6 weeks before realizing that was what was causing his mucous and flu like symptoms.