Food allergy (also called food hypersensitivity) refers to a type of physical reaction to food. Food reactions are classified into two categories: those that are the result of immune system stimulation and those that are not. Food allergy occurs when the immune system begins to overreact to ingredients that the pet has eaten with no problems in the past. Food intolerance occurs when what is eaten has a direct, negative effect on the stomach and/or intestines, such as spoiled meat, chewed up toys, food additives, and abrupt changes in diet. Food intolerance is not an immune reaction.