There's a lot of confusion about food allergies. Often a symptom related to the ingestion of particular food has nothing to do with the immune system and, therefore, is not an allergic reaction. Indigestion? Maybe. Lactose intolerance? Possibly. Food sensitivity? Could be. Several conditions can be mistaken for an allergic reaction.

• Lactose intolerance — Some people cannot tolerate dairy products because they lack normal quantities of the enzyme, lactase, needed to digest lactose (milk sugar). If lactose intolerance is your problem, you probably develop gas, cramps or diarrhea when you consume dairy foods. Some people are affected only when they have larger than usual amounts of these foods, and most lactose-intolerant people can tolerate hard cheeses, yogurt and sour cream without developing symptoms. Most people diagnose lactose intolerance without any scientific tests—they simply identify that their symptoms occur after meals that contain milk, and identify ways to limit symptoms by avoiding milk products. It is possible to test for lactose intolerance by monitoring the quantity of hydrogen gas in your breath after you consume milk; this test is not usually necessary, however.

• Asthma — People with asthma can develop attacks after eating foods containing sulfites, additives used to keep food looking fresh and to retard mold growth. Although the U.S. Food and Drug Administration now bans the use of sulfite sprays on fresh fruits and vegetables, they still are used in dried fruit and other foods. Sulfites also develop naturally in wine during fermentation.

• Reactions to specific foods — Many people can't eat beans, peas, broccoli or cabbage without developing uncomfortable intestinal gas. These “gas-producing” foods contain indigestible carbohydrates, so they can ferment in the colon. Other people develop indigestion and diarrhea after eating mushrooms or drinking certain wines. These foods contain sulfites, which may be the source of their irritation. While the symptoms sometimes are confused with food allergies, they aren't caused by an allergic reaction.

• Reactions to food additives — Some people are convinced that the food additive monosodium glutamate (MSG), which often is used in Chinese food, can bring on symptoms that include dizziness, sweating, ringing in the ears and a feeling of faintness. This "Chinese restaurant syndrome" is not a food allergy, and researchers have had great difficulty confirming that it is even a true reaction. A few studies have been done to try to demonstrate this reaction, but none has shown that people are more likely to react to MSG than they are to a placebo (sugar pill). This has been true even when the people in the study had previous reactions that they blamed on MSG.

• Food poisoning — Nausea, vomiting and diarrhea caused by food poisoning or by an infection, such as gastroenteritis, can be mistaken for a food allergy. Food poisoning stems from contamination with bacteria byproducts. Gastroenteritis is a viral infection by any of several different viruses that affect the digestive system. A bout of food poisoning mistakenly labeled as an allergy can affect dietary choices for years to come.

• Histamine toxicity — You can develop a headache and heat and flushing of the face after eating foods containing histamine, a natural substance involved in the allergic process. Foods high in histamine include cheese and some wines. Certain kinds of fish, particularly tuna and mackerel, produce histamine if they become spoiled. A reaction to histamine in spoiled fish is called “scombroid food poisoning.”