An allergic reaction happens when the body's immune system mistakes a part of the food for something harmful. Foods that cause the most problems for adults are peanuts, tree nuts (like walnuts or almonds), fish, and shellfish. Any food can cause an allergy, but some other "highly" allergenic foods are milk, eggs, soy products, and wheat. Allergic reactions usually happen within minutes to hours after eating an allergen.
Food intolerance is the digestive system's response to the part of a food or additive that can't be processed or that caused irritation. Intolerance could be from an additive, such as a sugar substitute in diet foods, that is poorly absorbed by the gut
The body's response to food intolerance is usually based on portion: a small amount provokes a small reaction; a large provokes a larger reaction. The time frame for problems is longer too. The body's reaction to food intolerance can take place immediately or over a few days.