How quickly your body reacts to a food allergen depends on your personal body chemistry and the speed at which the particular food breaks down. Type I reactions are immediate, meaning that symptoms appear within two hours of contact with the food. You only need to eat or touch a small amount of the allergen in order to have a Type I reaction, with symptoms affecting the skin, digestive tract and airways and possibly causing deadly anaphylactic shock. These reactions are much more common in children than in adults, and they are rarely caused by more than one or two foods in the same person.
In contrast, Type II reactions are delayed, with symptoms appearing between two and 24 (sometimes up to 72) hours after contact with the food allergen. Generally you need to eat larger amount of the food in order to have a Type II reaction, and it can be caused by up to 20 different foods in the same person. It's more difficult to diagnose Type II symptoms as stemming from an allergic reaction because of their delayed onset and the possibility that they may be caused by any number of different foods. Type II symptoms are not as easily recognized because they vary from physical reactions to behavioral ones like attention deficit, hyperactivity and insomnia. You may need multiple doctor's visits before diagnosis of a Type II food allergy is possible. Type II reactions are the most common type, and are very common in both adults and children. In fact, over 200 symptoms and 50 medical conditions have been reported to be caused, provoked or worsened by reactions to food allergens.
Type I reactions are caused by IgE antibodies in your immune system, which are trying to fight off food proteins they mistakenly think are harmful. Type II reactions are caused by T-cells that release chemicals, causing a chain reaction within your body, which takes longer; hence the difference in timing.