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

   People usually inherit the ability to form antibodies (IgE) against food. Those more likely to develop food allergies come from families in which allergies such as hay fever, asthma, or eczema are common.

   A person with a strong allergic tendency (atopic) must first be exposed to a specific food before IgE antibodies are formed. When this food is ingested for the first time, the body produces specific IgE against that food. The IgE then attaches to the surface of mast cells that contain chemical substances like histamine that are responsible for an allergic reaction.    The next time that particular food is eaten, the IgE on the mast cells are triggered to release histamine, which produces the symptoms of an allergic reaction.

What causes food allergy? 

   In adults, the most common foods that cause allergic reactions are: shellfish, such as shrimp, crayfish, lobster, and crab; nuts from trees, such as walnuts; fish; eggs; and peanuts. In children, the pattern is somewhat different from adults, and the most common foods that cause allergic reactions are eggs, milk, peanuts, and fruits, particularly tomatoes and strawberries.

    Children sometimes outgrow their allergies, but adults usually do not lose theirs. Also, children are more likely to outgrow allergies to cow's milk or soy formula than allergies to peanuts, fish, or shrimp. Adults and children tend to react to those foods they eat more often.