Food allergies

My son, now three years old has been diagnosed with food allergies as a result of a RAST test. Can you explain how food allergies are caused? Why is it that a period of avoidance of these foods could reverse the allergic condition?

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Robert Steele

Robert W. Steele, MD, is a board certified pediatrician at St. John's Regional Health Center in Springfield, MO. He graduated from medical... Read more

While our understanding of what causes the reaction of a food allergy is pretty good, why food allergies begin in some children and not in others is not well understood. In discussing food allergies, one must first distinguish the difference between food allergies and food intolerance. Not all reactions to food are food allergies. An allergy to food implies that the symptoms are caused by the body's immune system reacting to the food. An example of non-allergy is the "milk allergy" called lactose intolerance. There are true milk allergies, but the crampiness and diarrhea of lactose intolerance is caused by the relative lack of lactase and sucrase (enzymes which break down sugars) in the body rather than being a reaction of the immune system. This distinction between intolerance and allergy should be made by your doctor by taking a careful history and confirming the suspicion of allergy by tests.

Symptoms that suggest (but not confirm) allergy primarily involve the skin respiratory, or intestinal systems. Any combination of symptoms may be present after ingesting the food causing the allergy:

Respiratory:
Itching and swelling of the tongue, lips, and throat.
Wheezing, coughing, runny nose may also appear.
Skin:
Swelling around eyes, itching, and a rash. It may worsen a rash your child has had for a long time (e.g. eczema).
Intestinal:
Nausea, vomiting, diarrhea, and cramping.

Questions your doctor will want to know include:

  1. What foods do you think caused the reaction?
  2. How much of the food did your child eat?
  3. How much time elapsed between eating the food and developing the symptoms?
  4. Precisely what were the symptoms?
  5. Did similar symptoms ever develop when the food was eaten at other times?
  6. Does the reaction occur only when other factors, such as exercise, are a part of the picture?
  7. How long has it been since the last reaction?

When your doctor then suspects that there may be a food allergy, first a screening and then confirmatory tests must be done. The screening is primarily done by one of two tests:

  1. Skin-prick tests - These are just as they sound. A small amount of liquid containing the suspicious food (e.g. peanut) is placed on the skin. Then the skin is pricked. After 15 minutes, your doctor will look for a reaction.
  2. Radioallergosorbent test (RAST) - This is a blood test which identifies specific food IgE antibodies (part of the immune system which causes allergic reactions)

Once the screen and history have identified possible (and I stress possible) food culprits, a food challenge must be done. This challenge test may be simple or more involved depending upon the type and number of implicated foods. This is done in the doctor's office after the child has avoided the food for 1-2 weeks. If the challenge is negative, it means either the screening test was falsely positive or the allergy is so small as to be insignificant. The only exception to doing this confirmatory test is if the child had a life-threatening reaction to the suspected food.

Once the diagnosis of food allergy is made, the hard part begins in finding out where that food and its derivatives may be hiding (e.g. canned soups, chili at the fast food restaurant, etc.) Your doctor with the help of a nutritionist can help you here. Strict avoidance of these foods is essential because if your child is going to outgrow this allergy, he will do so faster by never taking in these foods.

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