Food allergies are the result of a repeated mistake by your immune system. Once your body accidentally tags a certain protein as harmful, it will continue to think that substance poses a danger -- even though it doesn't. When you eat anything with that protein in it, your body overreacts and sends out antibodies called immunoglobulin E. The antibodies then tell your mast cells to handle the problem; the mast cells release an army of chemicals to fight off the allergen. One of these chemicals, histamine, is the chemical responsible for most of your symptoms.
Food allergies affect children more often than adults. Some of the most common food allergies in children are milk allergies, egg allergies, shellfish allergies, soy allergies, wheat allergies, peanut allergies and tree nut allergies. Kids tend to outgrow allergies to milk and eggs by age three or four, but shellfish and peanut allergies are often life-long.