Some children can enjoy a peanut butter and jelly sandwich every day. Others cannot even be in the same room with a peanut butter sandwich. The reason is that some children are highly allergic to peanuts. Still, peanuts are not the only source of food allergies. In fact, there are many other foods that, while tasty and nutritious for most people, may actually cause serious reactions in a small number of others.

One of about every 25 children in the United States experiences some kind of allergic reaction to food every year. Allergies are less common among teenagers and adults.  The symptom of a food allergy can be as minor as a case of hives or a tingling sensation in the mouth. Itchy eyes, vomiting, and diarrhea can also be signs of food allergy. The types of symptoms are person-specific rather than food-specific.  Two people may be allergic to fish but show much different allergy symptoms.  In contrast to illness that is caused by eating contaminated food, allergy symptoms can occur within minutes after eating the allergy-causing food. Occasionally, people with severe allergies may experience anaphylaxis,  or an anaphylactic reaction, if they inadvertently eat the food that triggers their allergy.  Anaphylaxis is a life-threatening condition characterized by a drop in blood pressure,  rapid swelling in the face or throat region,  and difficulty breathing.  Each year in the United States, 30,000 people are rushed to emergency rooms because of anaphylactic reactions to food.

The ABCs of Food Allergies
Our immune system is made up of a complex set of organs, cells, and chemicals that protect our bodies from “foreign” substances.

Cells of the immune system circulate throughout the body, ready to attack bacteria,  viruses,  or toxic chemicals that find their way inside. We usually think of our immune systems as helping us to fight of the type of organisms that cause sore throats, wound infections,  or even food poisoning.  Our digestive systems have developed a special form of immune system that is able to recognize most substances in the foods we eat as safe, even though they are otherwise foreign to the body. The normally functioning immune system does not mount an immune response when these foods pass through the intestinal tract. The foods are broken down by the digestive system, and essential nutrients are absorbed by the body, all without triggering a reaction by the immune system.

A food allergy is a reaction by the body’s immune system against a food substance that most people can eat with no ill effect. The particular chemical substance in the food that causes the immune system to react is called an allergen.  For example, peanuts contain several forms of proteins, in addition to fats and micronutrients. The chemical structures of some peanut proteins are recognized as allergens by some people’s immune systems. As a result,  these people develop an allergic reaction if they ingest peanuts.

At birth,  an infant’s immune system is not fully developed. Over the child’s first three years,  the child’s immune system develops as it learns to tolerate new foods introduced into the diet.

But sometimes, something goes wrong during this development process-the immune system may start reacting to a particular food substance as being foreign. When this happens, the immune cells mount a response to this foreign invader by producing specialized proteins called antibodies.  One type of antibody that plays an important role in producing the allergic response is called IgE. These IgE antibodies cause the release of biologically active chemicals that spread throughout the body.  This is part of the body’s normal response to protect itself against threatening substances. But in the case of allergies, the food substance is actually harmless, and the immune response is inappropriate. The chemicals produced by the immune system cause the damage that brings on the symptoms of food allergy. The symptoms may occur in the skin, respiratory tract, or gastrointestinal tract, depending on the individual.

Until a few decades ago,  many doctors did not believe that babies could suffer from food allergies.  Much has been learned since then,  and researchers continue to learn more about what causes food allergies and how they can be prevented.  We now know that the genetic codes inherited from the parents play a role in whether a particular baby’s developing immune system will show normal or allergic responses to food.  Babies born to parents with allergies are more likely to develop food allergies.

In addition, a mother’s diet during her pregnancy and the timing of introduction of new foods into the baby’s diet may play a role in training the immune system to tolerate changes in diet. Researchers continue to study the process by which infants learn to tolerate new foods in order to tell new mothers how best to prevent the occurrence of allergies in their infants. The American Academy of Pediatrics recommends that infants at high risk of allergies (for instance,  those whose parents or siblings have allergies)  be exclusively breast-fed for at least four months. They further recommend that parents delay the introduction of solid foods until the infant is four to six months of age.

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Terry L. Smith
TERRY L.  SMITH is a biostatistician and science writer who
lives in Lawrence, Kansas. She has an M.S. in biometry from the
University of Texas School of Public Health. Smith is the author of
numerous books and articles relating to human health, including
Asthma in Chelsea House’s Genes and Disease series.