An allergy has the potential to affect the whole body. Sometimes a severe reaction. just like when you're sick with an infection your body will make stuff to fight it off, when you have an allergy your body sees it as an invasion and tries to fight it. An intolerance generally involves just the digestive tract and is caused mainly by a lack of a specific enzyme that breaks down a specific food. The symptoms are also generally different, Allergy: hives, itchy, swelling, difficulty breathing... Intolerance: diarrhea or constipation, nausea, vomiting... Allergies are life threatening whereas an intolerance is generally not. I hope that helps

In an allergy your body tries to fight off the offending food, it's more immediate, and can be life threatening really quickly. In an intolerance your body is unable to digest the food, and sometimes (like with Celiac disease) your immune system actually damages your intestines to try to keep you from absorbing the food. Food intolerances can cause you to be malnourished (from damaged intestines) and cause a host of other symptoms due to not absorbing food.