Food allergies and food intolerances are often confused, but they're not the same thing. The real (biological) difference is that a food allergy is related to your immune system whereas food intolerance is related to your digestive system. Many more people have food intolerances than have food allergies. When you're allergic to a food, it means that when you eat it, your immune system misidentifies it as dangerous and sends out antibodies to fight off its proteins. You suffer allergic symptoms as a result of the battle between the allergen and the immune system. Meanwhile, if you have food intolerance, your body has trouble digesting certain foods. Your inability to break down particular proteins results in unpleasant side effects.
The symptoms of a food allergy are generally the result of the compound histamine; depending where in your body histamine is released, you'll have symptoms in different places. Typical symptoms are hives, rashes, diarrhea, nausea, vomiting, swelling, coughing and sneezing. Food intolerance can also cause nausea, diarrhea and vomiting; in addition, food intolerance can cause gas, heartburn, headaches and irritability.
Food allergies are often hereditary, but they can be outgrown. Food intolerance -- lactose intolerance, for example -- often means you're missing certain enzymes that are necessary to properly digest particular proteins. Some food intolerances are reactions to chemicals added to foods for flavoring, coloring or preserving. When it comes to a food allergy, portion size is generally unrelated. Even a bit of the allergen is enough to trigger a reaction, therefore, people with food allergies are told to avoid the foods they're allergic to altogether. On the other hand, food intolerance is often related to portion size. Sometimes people with food intolerance can handle small amounts of the food they're sensitive to.