True food allergies are caused by an immune system response to a specific allergen in the food. Reactions occur quickly and every time the food is eaten. Most often, these reactions involve hives or swelling; vomiting, belly pain and trouble breathing may also occur.
With food intolerances, the vast majority are not related to the immune system. However, food intolerances can cause symptoms similar to food allergies, such as diarrhea, nausea and belly pain.
An allergy or gastroenterology specialist can help you distinguish between a food allergy and an intolerance.