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Food Allergy
IntroductionFoods can cause a number of reactions, not all of which are allergic. Anyone can experience an adverse reaction to a food. The types of adverse reactions are as follows: Adverse Reactions to FoodsFood Intolerance - where the immune system is not involved in the reaction
Food Allergy - when the immune system is involved in the reaction
Food Allergy - Facts and Figures
Allergic Reactions to FoodsFoods can trigger a variety of reactions involving the immune system. The list includes the following:
Systemic anaphylaxis can be caused by any food. There tends to be associations with different ages. An infant or young child may tend to have anaphylaxis to peanuts, eggs, cow's milk, and fish. Adults have a slightly different list that still include peanut and fish, but also includes shellfish and tree nuts. Local anaphylaxis is a misnomer- it is the complaint of localized swelling of the mouth, an itchy tongue, or hoarseness. It has been seen in association with fruits and vegetables. This is never a serious reaction and is often referred to as the oral allergy syndrome. There seems to be an association between the offending food and the coexistence of pollen allergy. Patients with ragweed sensitivity may have complaints after eating watermelon or bananas. Those with birch tree pollen sensitivity may react to apples, carrots, potatoes, and hazelnuts. The food associated exercise-induce anaphylaxis syndromes are rare and confusing. The affected patient may be able to exercise without any problems and will be able to eat the suspected food and experience nothing. However, if the triggering food is eaten within a few hours of exercise, there may be anaphylaxis. There are well-documented cases of this occurring after celery, shrimp, oysters, peaches, wheat, chicken, and a number of other foods. The thought of vigorous exercise after eating some of these foods is an issue by itself! Atopic dermatitis may also have a food as a contributor to the severity of the disease. The food may not be the only trigger. These patients react to a variety of stimuli, but identifying a food trigger may help overall control. This is most frequently seen in infants and young children. It usually appears after the 3rd month of life. The cause-effect relationship of ingestion and skin symptoms is usually not obvious to the parents. Six foods account for the majority of cases- egg, wheat, soy, milk, peanut, and fish. Although the allergy tests may be positive for a number of foods, it is usually only one that is the culprit. These children show many false positive skin tests to foods, but for most of these, it is not clinically relevant. Oddly enough we get better results from the negative as opposed to the positive skin test. The negative predictive value is 6%- if the test is negative there are only 6 chances out of 100 (6%) that the food is the cause. A positive skin test is a 50/50 proposition. It may or may not help. We use it as a starting point to direct avoidance diets. Urticaria and angioedema may have a food as the trigger. This is usually seen in the acute cases- hives that have been present for only a few days. Chronic urticaria and the deeper tissue swelling known as angioedema are rarely if ever due to foods. The history is helpful. Skin testing has a low rate of return (please see the section on hives). There are a variety of food reactions that affect the gastrointestinal tract. The list includes:
The eosinophilic gastrointestinal syndromes seem to have a food associated; unfortunately, the reaction is not one that is predicted by skin testing. One of the more frustrating aspects of this condition is the lack of any test to determine what the cause truly is. The immune system is involved, but it does not leave a marker that is easy to use to make any predictions. Food and respiratory illnesses has been a very hot topic. For years it was believed that food, especially cow's milk was a major cause of nasal symptoms and a cause of asthma. There are many recent studies that have decreased the role of foods in respiratory illnesses. In large series of observations on what happens to children when undergoing a food challenge, isolated wheezing was seen in only 2% of those who thought a food was involved. Milk is the food we hear about in the clinic, however some recent rather exciting work has implicated another food- egg. Eggs did not cause over wheezing when ingested. They were however associated with increased airway nonspecific reactivity. They made it easier for other things to trigger an event. More work is needed in this area. We will keep you posted. Non-allergic Reactions to FoodsThere has always been concern that foods were related to headaches. This may be the case, however the mechanism is not an allergic one. They do not cause headaches through an immune mechanism. Skin testing cannot predict these reactions. Remember, foods may contain pharmacologic agents that may cause headache in susceptible individuals. Prime examples of foods that may cause pharmacologic reactions are:
There is also concern that behavior problems are due to food allergy. This is an area that has been extensively evaluated. The behavior problems are not mediated by type I immune mechanisms. They are not IgE mediated problems. Skin tests and blood tests for allergy look to see if the patient makes a specific antibody- IgE. The illnesses due to IgE reactions are well defined. Food preservatives and dyes have also been an area of concern. Unfortunately these chemicals do not react by a Type I immune response. There are no skin test materials available for testing these products, nor are there any blood tests. The Diagnosis of Food AllergyThe history is most important. The nature of the reaction, the frequency and importantly the cause-effect relationship needs to be sorted out. The physical examination also is necessary to direct the next step- testing. Skin tests or blood tests are done to confirm the suspected diagnosis. The skin test by itself does not make the diagnosis. The skin test is an adjunct. Challenges are the gold standard for the diagnosis. Once the suspected food is identified, the therapeutic plan is then formulated. Treatment of Food AllergyNothing will work better than avoidance. Families need to be shown how to read food labels and how to identify the variety of ways a food can be labeled. In our clinic we go over the many ways of identifying eggs, or milk, or soy in a product. Families are advised to find products that are free of the allergen and use them. We also try to have them enroll in the Food Allergy Network. This is a fantastic organization. They provide warnings about contamination issues. Help with food substitutes and offer educational material to help schools, playgroups, and relatives understand the problem. The most important rule regarding avoidance is that if you don't know, don't eat it. If anaphylaxis is the issue- epinephrine is made available and instructions are given as to when and how to use it. Trainer devices are available in our clinic. When a family leaves with a prescription for epinephrine, they also leave knowing how to use it. If needed the trainer devices are sent home so others who are in contact with the child are comfortable with its use. Medical alert bracelets help. They serve as a warning to those who may offer potentially life-threatening treats. There are no drops or shots that have been shown to work and are safe. We can help with avoidance and treat the episodes. Repeat skin testing can be done after a year or two of avoidance. If there has been strict avoidance yet the skin test is still positive, a challenge can be offered. Challenges are dependent upon the nature of the reaction. They are not done for anaphylaxis. Support GroupsWe have helped with the establishment of a food allergy support group in the Evansville area. The credit goes to the mother of our patient who has done a wonderful job in running this group. The Food Allergy Network is also a wonderful group. |
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