Food allergies affect only a few individuals in the population. Food allergies involve an abnormal immunologic response to a particular food or food component, usually a naturally occurring protein component of the food. The same food or food component would be safe and nutritious for the vast majority of consumers. The most common form of true food allergies occurs through an antibody-mediated mechanism involving immunoglobulin E (IgE).
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.
Allergen-specific IgE antibodies, once produced, attach themselves to the membrane surfaces of 2 types of specialized cells: (1) mast cells, which are found in many different tissues, and (2) basophils, which are found in the blood. In this so-called “sensitization” process, the mast cells and basophils become sensitized and ready to respond to subsequent exposure to that specific food allergen. Adverse reactions will then occur on any subsequent exposure to the specific allergenic protein or some closely related protein. Sensitization to a particular allergen distinguishes allergic individuals from nonallergic individuals.
Once the mast cells and basophils are sensitized, subsequent exposure to the allergen results in the allergen cross-linking 2 IgE molecules on the surface of the mast cell or basophil. This interaction between the allergen and the allergen-specific IgE triggers the release of a host of mediators of allergic disease that are either stored or formed by the mast cells and basophils. Several dozen different mediators have been identified, including histamine, prostaglandins, and leukotrienes. These mediators are responsible for the symptoms associated with allergies through interactions with receptors in various target organs of the body. 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 mechanism of IgE-mediated reactions 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 rather wide variety of symptoms can be associated with IgE-mediated food allergies (Table 1). Fortunately, most food-allergic individuals suffer from only a few of the many possible symptoms.
Most of the symptoms of IgE-mediated food allergies are not particularly definitive, which can make clinical diagnosis rather difficult. For example, the gastrointestinal manifestations of food allergies can also be associated with many other foodborne illnesses and a variety of other diseases as well. Additionally, there are millions of asthmatics, but only a few are allergic to foods.
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 depend 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.
The most common allergenic foods are peanuts, tree nuts (almonds, walnuts, pecans, cashews, etc.), soybeans, cow's milk, eggs, fish, crustacea (shrimp, crab, lobster, etc.), and wheat. Allergies to crustacean shellfish and peanuts are the most common food allergies in the United States. Cow's 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 that 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 rarely 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% to 8% in infants to perhaps 2% to 4% in adults. Thus, many infants and young children outgrow their IgE-mediated food allergies through the development of oral tolerance. Allergies to some foods, such as cow's milk and eggs, are more frequently outgrown than allergies to other foods, such as peanuts.
The diagnosis of food allergies is typically approached in stepwise fashion. The diagnosis of food allergies by an allergist is often critical because parental diagnosis and self-diagnosis are often incorrect, leading to identification of the wrong foods or the identification of too many foods as allergens. The diagnosis begins with the physician taking a careful history of the patient's adverse reactions and taking note of the foods eaten immediately before the onset of symptoms; the amount of the various foods consumed; the type, severity, and consistency of the symptoms; and the time intervals between eating and the onset of symptoms. Sometimes, histories are needed from several episodes to reach a probable diagnosis. Challenge tests with the suspected food(s) may be used to establish with certainty the role of a specific food in the reaction. History alone can be sufficient to make the diagnosis in some situations if the cause-and-effect relationship is particularly compelling. The diagnosis of an IgE-mediated mechanism can be made with either the skin prick test or the radioallergosorbent test, a blood test for the presence of specific IgE antibody.
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, low amounts of the offending food can provoke reactions. Thus, the construction of a safe and effective avoidance diet can be quite difficult. Ingredients derived from commonly allergenic foods must also be avoided in many cases, such as casein or whey from milk or semolina from wheat. The sources of ingredients derived from commonly allergenic foods must be labeled, and food-allergic consumers are advised to avoid them even in situations where the risks may be quite small. Source labeling is not required for refined oils from allergenic sources, such as peanut oil and soybean oil, because these ingredients do not contain enough of the allergenic protein to provoke reactions. The careful reading and complete understanding of food labels is critical to the implementation of safe and effective avoidance diets. Of course, the manufacturers of packaged foods have the responsibility to ensure that the label statements on packages are accurate. Occasionally, errors are made by food processors that result in the presence of undeclared residues of allergenic foods in a packaged food. The contamination of one food with another from the use of shared food processing equipment is one of the most common errors occurring in food manufacturing. 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 (utensils, cooking surfaces, pots, and pans, etc.). Additionally, the accurate identification of all of the ingredients in food service and restaurant meals can sometimes be quite difficult, and such foods are not labeled. As a result, many inadvertent exposures occur for allergic consumers who are attempting to avoid their offending food(s).
Cross-reactions are another perplexing issue for food-allergic consumers as they attempt to develop effective avoidance diets. Cross-reactions can occur but do not inevitably occur between closely related foods. For example, many individuals are allergic to peanuts, but most of these individuals are not allergic to other legumes, such as soybeans, peas, and green beans. A few of these individuals are cross-reactive with one or more other legumes. Alternatively, cross-reactions frequently occur among the various crustacea (shrimp, crab, lobster, and crayfish). Cow's milk and goat's milk invariably cross-react, as do the eggs of various avian species. Cross-reactions can also occur between foods and other environmental allergens. The most common examples are the cross-reactions that occur between some fresh fruits and vegetables and certain pollen allergies in some individuals and the cross-reaction that occurs in a few individuals with allergies to natural rubber latex with several foods, including bananas and kiwis.
Pharmacological approaches can be used to treat the symptoms of allergic reactions. In particular, epinephrine (also known as adrenalin) is prescribed for individuals who experience life-threatening food allergies. The early administration of epinephrine after inadvertent exposure to the offending food can be lifesaving for such patients. Antihistamines can also be used to treat the less serious symptoms of food allergies.
Infants born to parents with histories of allergic disease are much more likely than other infants to develop food allergies. Prevention of the development of food allergies in such infants is quite difficult. The avoidance of commonly allergenic foods such as cow's milk, eggs, and peanuts, primarily through breast-feeding, appears to delay but not prevent the development of food allergies.
A few specialized hypoallergenic foods are available in the marketplace. These foods are intended for infants who have developed allergies to infant formula made with cow's milk. The most effective hypoallergenic infant formulae are based on extensively hydrolyzed casein. Although casein is a common milk protein and a major milk allergen, the hydrolysis of its peptide bonds renders it safe for cow's milk-allergic infants.
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