Let's start by looking at food allergy.
Food allergy is usually easy to recognise.
An allergy to a food gives rise to symptoms very quickly, generally within minutes of eating or even touching the food. The symptoms are often severe, and may even be life-threatening. Food allergies are usually due to the protein component of the offending food. For some reason some of the food protein is absorbed from the intestine intact, instead of being digested as most proteins are. Once the intact protein is in the blood stream, it is recognised as a foreign protein to the body, or in other words as an antigen .