A food intolerance means either the body cannot properly digest the food that is eaten, or that a particular food might irritate the digestive system. Symptoms of food intolerance can include nausea, gas, cramps, abdominal pain, diarrhea, irritability, nervousness, or headaches.
A food allergy happens when the body's immune system, which normally fights infections, sees the food as an invader. This leads to an allergic reaction — a response from the immune system in which chemicals like histamine are released in the body. The reaction can cause symptoms like breathing problems, throat tightness, hoarseness, coughing, vomiting, abdominal pain, hives, swelling, or a drop in blood pressure.
Even if previous reactions have been mild, someone with a food allergy is always at risk of the next reaction being life-threatening. Eating a microscopic amount of the food, or sometimes even touching or inhaling it, could lead to anaphylaxis. So anyone with a food allergy must avoid the problem food(s) entirely and always carry emergency injectable epinephrine.
Many people with food sensitivities, on the other hand, can ingest a small amount of the bothersome food without a problem.