Differentiate between food allergy and food intolerance.

A food allergy occurs when the body over-reacts to a normally harmless protein and perceives it to be an allergen. The body then produces antibodies that will trigger an immune cascade when the allergen presents itself in a subsequent exposure. The reactions can be anything from hives and a rash to swelling (particularly dangerous if it affects the respiratory system), irregular heartbeat and death. Symptoms may not occur right away and do not occur upon first exposure to the food.
Food intolerance does not involve the immune system. It occurs in people who lack certain digestive enzymes and so complete breakdown of the food is not accomplished. The food then moves to the large intestine where bacteria attempt to break it down, producing a large amount of gas in the process.