Either food allergy or food intolerance affects nearly everyone at some point. When people have an unpleasant reaction to something they ate, they often think that they have an allergy to the food. Actually, however, only about 1% of adults and 3% of children have clinically proven true allergic reactions to food.

This difference between the prevalence of clinically proven food allergy and the public's perception of the problem is due primarily to misinterpreting food intolerance or other adverse reactions to food as food allergy. Food allergy is an abnormal response to food that is triggered by a specific reaction in the immune system and expressed by certain, often characteristic, symptoms. Other kinds of reactions to foods that are not food allergies include food intolerances (such as lactose or milk intolerance), food poisoning, and toxic reactions. Food intolerance also is an abnormal response to food, and its symptoms can resemble those of food allergy. Food intolerance, however, is far more prevalent, occurs in a variety of diseases, and is triggered by several different mechanisms that are distinct from the immunological reaction responsible for food allergy.

People who have food allergies must identify and prevent them because, although usually mild and not severe, these reactions can cause devastating illness and, in rare instances, can be fatal.

How do allergic reactions to food occur?

The allergens in food are those components that are responsible for inciting an allergic reaction. They are proteins that usually resist the heat of cooking, the acid in the stomach, and the intestinal digestive enzymes. As a result, the allergens survive to cross the gastrointestinal lining, enter the bloodstream, and go to target organs, causing allergic reactions throughout the body. The mechanism of food allergy involves the immune system and heredity.

Immune system: An allergic reaction to food involves two components of the immune system. One component is a type of protein, an antibody called immunoglobulin E (IgE), which circulates through the blood. The other is the mast cell, a specialized cell that is found in all tissues of the body. The mast cell is especially common, however, in areas of the body that are typical sites of allergic reactions, including the nose and throat, lungs, skin, and gastrointestinal tract.

Heredity: The tendency of an individual to produce IgE against something seemingly as innocuous as food appears to be inherited. Generally, people with allergies come from families in which allergies are common -- not necessarily to food but perhaps allergies to pollen, fur, feathers, or drugs. Thus, a person with two allergic parents is more likely to develop food allergies than someone with one allergic parent.

Mechanism: Food allergy is a hypersensitivity reaction, meaning that before an allergic reaction to an allergen in food can occur, a person needs to have been exposed previously, that is, sensitized, to the food. At the initial exposure, the allergen stimulates lymphocytes (specialized white blood cells) to produce the IgE antibody that is specific for the allergen. This IgE then is released and attaches to the surface of the mast cells in different tissues of the body. The next time the person eats that food, its allergen hones in on the specific IgE antibody on the surface of the mast cells and prompts the cells to release chemicals such as histamine. Depending upon the tissue in which they are released, these chemicals cause the various symptoms of food allergy.