You will not find a better resource online or off than the Food Allergy and Anaphylaxis Network. Explore their site at http://www.foodallergy.org. Join and Donate. They were my number one source and very much respected.
Here is their definition:
A food allergy is an immune system response to a food that the body mistakenly believes is harmful. Once the immune system decides that a particular food is harmful, it creates specific antibodies to it. The next time the individual eats that food, the immune system releases massive amounts of chemicals, including histamine, to protect the body. These chemicals trigger a cascade of allergic symptoms that can affect the respiratory system, gastrointestinal tract, skin, and/or cardiovascular system. Scientists estimate that approximately 12 million Americans suffer from food allergies.