A food allergy, or hypersensitivity, is an abnormal response to a food triggered by the immune system.

While many people often have gas, bloating or another unpleasant reaction to something they eat, this is not an allergic response.

Such a reaction is thought to not involve the immune system and is called "food intolerance."

Only about 1.5 percent of adults and up to 6 percent of children younger than 3 years in the United States--about 4 million people--have a true food allergy, according to researchers who have examined the prevalence of food allergies.

It's critical for people who have food allergies to identify them and to avoid foods that cause allergic reactions. Some foods can cause severe illness and, in some cases, a life-threatening allergic reaction (anaphylaxis) that can constrict airways in the lungs, severely lower blood pressure, and cause suffocation by the swelling of the tongue or throat.