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Food Allergies and Sensitivities

Presenter: Steve L. Taylor, Ph.D.
University of Nebraska
Department of Food Science & Technology
Food Allergy Research & Resource Program Lincoln, NE

This SciTalk made possible through an educational grant from the Institute of Food Technologists

Background
Discussion


The ancient Roman poet and philosopher, Lucretius, was quoted as saying: "One man's food may be another man's poison". This quote applies rather nicely to food allergies and sensitivities. While most foodborne illnesses have the potential to affect everyone in the population, food allergies and sensitivities affect only a few individuals in the population. The most serious type of illness among these individualistic adverse reactions to foods are the true food allergies that are the subject of this Sci-Talk. Food allergies involve an abnormal response of the immune system to a particular food or food component, usually a naturally-occurring protein component of the food, which is safe and nutritious for the vast majority of consumers.

The most common and severe form of food allergy involves the development, in susceptible individuals, of IgE antibodies that are able to recognize and react with a protein component of a specific food. In IgE-mediated food allergies, allergen-specific IgE antibodies are produced in the body in response to exposure to a food allergen, usually a protein. These IgE antibodies are highly specific and will recognize only a specific portion of the protein that they are directed against. Occasionally, IgE antibodies produced against one particular protein in a specific food will confer sensitivity to another food either because the food is closely related or because it shares a common segment with the allergenic protein. Some food proteins are more likely to elicit IgE antibody formation than others. While exposure to the food is critical to the development of allergen-specific IgE, exposure will not invariably result in the development of IgE antibodies even among susceptible people. Many factors including the susceptibility of the individual, the immunogenic nature of the food and its constituent proteins, the age of exposure, and the dose, duration, and frequency of exposure are likely to influence the formation of allergen-specific IgE antibodies.

Once they are produced by immune cells in the body, allergen-specific IgE antibodies attach to the outer membrane surfaces of two types of specialized cells: MAST CELLS, which are found in many different tissues, and BASOPHILS, which are found in the blood. In this so-called "sensitization" process, the mast cells and basophils are sensitized and ready to respond to subsequent exposure to that specific food allergen. However, the sensitization process itself does not result in any symptoms. No adverse reactions will occur without subsequent exposure to the specific allergenic protein or some closely related protein. Once the mast cells and basophils are sensitized, subsequent exposure to the allergen results in the allergen cross-linking with IgE antibodies on the surface of the mast cell or basophil membrane. This interaction between the allergen and the allergen-specific IgE triggers the release of a host of chemical mediators of allergic disease which are either stored or formed by the mast cells and basophils. Histamine is one of the primary mediators of IgE-mediated allergies and is responsible for many of the early symptoms associated with allergies. The interaction of a small amount of allergen with the allergen-specific IgE antibodies results in the immediate release of comparatively large quantities of the various mediators into the bloodstream and tissues. Thus, exposure to extremely small amounts of allergens can elicit symptoms. This IgE mechanism is involved in many different types of allergies to foods, pollens, mold spores, animal danders, bee venom, and pharmaceuticals. Only the source of the allergen is different.

IgE-mediated food allergies are sometimes called immediate hypersensitivity reactions because of the short onset time (a few minutes to a few hours) between the ingestion of the offending food and the onset of symptoms. Since the mediators released from the mast cells and basophils can interact with receptors in a number of different tissues in the body, a wide variety of symptoms can be associated with IgE-mediated food allergies. The most common symptoms associated with food allergies are those involving the skin (hives, eczema, itching) and the gastrointestinal tract (vomiting, diarrhea). Respiratory symptoms (rhinitis, asthma) are less frequently involved with food allergies than with various inhalants such as pollen and animal dander allergies. However, asthma is a very serious, though uncommon, respiratory manifestation of food allergies. Fortunately, most food-allergic individuals suffer from only a few of the many possible symptoms. Anaphylactic shock is, by far, the most serious manifestation of food allergies. Anaphylactic shock involves gastrointestinal, cutaneous, and respiratory symptoms in combination with a dramatic fall in blood pressure and cardiovascular complications. Death can ensue within minutes of the onset of anaphylactic shock. Fortunately, very few individuals with food allergies are susceptible to such severe reactions after the ingestion of the offending food.

The severity of an allergic reaction will be dependent to some extent on the amount of the offending food that is ingested. Severe reactions are more likely to occur when an allergic individual inadvertently ingests a large amount of the offending food, especially if that individual happens to be exquisitely sensitive. However, exposure to even trace quantities can elicit noticeable reactions due to the large release of mediators. Some food-allergic individuals are at risk of death if they inadvertently ingest their particular offending food. Recent research on deaths associated with food allergies indicates that teenagers are at greatest risk of death because they tend to take risks with the foods that they consume, fail to carry medications that might reverse the severe reactions, and no longer are reliant on their parents for their safety. The most common allergenic foods are peanuts, tree nuts (walnuts, almonds, etc.), cashews, soybeans, cows' milk, eggs, fish, crustacea, and wheat. Peanuts and the various tree nuts cause most of the deaths but deaths have also been attributed to milk, eggs, soybeans, fish, and crustacean shellfish (shrimp, lobster, etc.).

Peanut allergy is the most common food allergy especially in the U.S. where peanuts are a popular dietary item and peanut butter is introduced at an early age. Throughout the world, cows' milk allergy is the most common food allergy among infants due to the widespread ingestion of milk during the first months of life. Any food which contains protein has the potential to elicit an allergic reaction in someone. The most common allergenic foods tend to be foods with high protein content that are frequently consumed. The exceptions are beef, pork, chicken, and turkey which are uncommonly allergenic despite their frequent consumption and high protein content.

The prevalence of IgE-mediated food allergies is not precisely known. The overall prevalence of food allergies in the developed countries of the world ranges from 4-8% in infants to perhaps 1% in adults. Thus, many infants and young children outgrow their IgE-mediated food allergies. The reasons for the development of tolerance to previously allergenic foods are not fully understood. Allergies to some foods such as cows' milk and eggs are more frequently outgrown than allergies to other foods, such as peanuts. The specific avoidance diet is the primary means of treatment for IgE-mediated food allergies. For example, if allergic to peanuts, don't eat peanuts. With IgE-mediated food allergies, very low amounts of the offending food can be tolerated by most allergic individuals. However, severe reactions have resulted from the trace amounts that would result from the use of a shared eating utensil or cooking equipment such as using a knife that had been previously used with peanut butter. Thus,the construction of a safe and effective avoidance diet can be quite difficult. Food-allergic patients must have considerable knowledge of food composition. For example, casein, whey, and lactose are common food ingredients that are derived from cows' milk. These milk ingredients would likely be hazardous for milk-allergic individuals. The ingredient must contain the specific allergenic protein to be hazardous to the allergic consumer. For example, peanut oil and soybean oil, despite being derived from allergenic sources, do not contain protein and would not be hazardous for peanut-allergic or soy-allergic individuals unless the oils had become contaminated during use. The careful reading and complete understanding of food labels is critical to the implementation of a safe and effective avoidance diet. Of course, the manufacturers of packaged foods have the responsibility to assure that the label statements on packages are accurate. However, restaurant and other food service meals can present an even bigger challenge for food-allergic individuals. Residues of allergenic foods can arise from the use of shared food preparation equipment.

Many inadvertent exposures occur among allergic consumers who are attempting to avoid their offending food(s). Individuals affected by severe food allergies should carry emergency medications to counteract a reaction in case their attempts at avoidance are not entirely successful. Self-injectable epinephrine or adrenalin (sometimes referred to as a bee sting kit) can be a life-saving drug for such individuals when the offending food is inadvertently ingested. Parents and friends of individuals with severe food allergies should learn how to administer the epinephrine also because immediate administration is advantageous.

Recently, some concerns have been expressed regarding the potential allergenicity of genetically modified foods. Such foods contain novel genes and the novel proteins that are expressed from those genes. Obviously, if a gene was selected from a peanut and cloned into another food crop such as tomatoes, then the possibility would exist that this gene might code for an allergenic peanut protein. However, in commercial practice, genes are usually selected from biological sources that have no history of causing allergies. In such circumstances, the risk of developing a novel allergenic food would be very small. Even so, developers of such modified crops are expected to test the novel proteins for their potential allergenicity.

BIBLIOGRAPHY

Brostoff, J., and Challacombe, S.J., eds (2002). "Food Allergy and Intolerance", 2nd ed. Saunders, London.

Metcalfe, D.D., Sampson, H.A., and Simon, R.A., ed. (1991). "Food Allergy-Adverse Reactions to Foods and Food Additives". Blackwell Scientific Publications, Boston.

Taylor, S.L., and Hefle, S. L. (2002). Allergic reactions and food intolerances. in "Nutritional Toxicology", 2nd ed. (F.N. Kotsonis, and M. Mackey, eds.). Taylor and Francis, New York, p. 93.

Taylor, S. L., Hefle, S. L., and Munoz-Furlong, A. (1999). Food allergies and avoidance diets. Nutrition Today 34:15.

Zeiger, R.S., and Heller, S. (1995). The development and prediction of atopy in high-risk children: follow-up at age seven years in a prospective randomized study of combined maternal and infant food allergen avoidance. J. Allergy Clin. Immunol. 95:1179.

TABLE 1. Symptoms of IgE-Mediated Food Allergies

Gastrointestinal symptoms Cutaneous symptoms
Vomiting Urticaria (hives)
Diarrhea Dermatitis
Nausea Angioedema
Respiratory symptoms Other symptoms
Rhinitis Anaphylactic shock
Asthma Laryngeal edema

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