50 90% of self reported food allergies are not allergies, severe food allergies do exist. They can have a sudden onset, and be fatal in minutes. Prior reactions dont predict future reactions. The only way to prevent reactions in those with a history of anaphylactic allergies is strict and complete avoidance of the allergen. Other types of reactions, such as celiac disease (an immune disorder triggered by gluten), require allergen avoidance, but are not immediately life threatening like allergies can be.