Allergies are an over-reaction of the body's immune system to a definite component, usually a protein (allergens like some foods, pollens, house dust, animal hair or moulds) The definition of ‘allergy’ means that the immune system has responded to a harmless substance as if it were toxic and creates a defense system (antibodies) to fight it. If you have a food allergy, eating even the smallest amount of that food may trigger a serious reaction. On the contrary food intolerance does not involve the immune response. It occurs when something in a food irritates a person's digestive system or when a person is unable to properly digest or breakdown, the food. With food intolerance, you may be able to eat small amounts of problem foods without a reaction. Food intolerance doesn't involve the immune system but can cause some of the same gastrointestinal symptoms as food allergy. When a person has allergy, his body produces excessive mucous, the bacteria in the mouth use the mucus as a food source and begin to extract sulfur compounds from the amino acids that make up the proteins found in all this excess mucus. The postnasal drip in combination with the bacteria creates a foul smell and taste. Saliva naturally washes away the oral producing bacteria, but the use of antihistamines dries the mouth.