A food allergy is defined as "an adverse health effect arising from a specific immune response that occurs reproducibly on exposure to a given food" (Boyce et al., 2010, p. 8). The most common type of food allergy is immunoglobulin-E (IgE) mediated which is considered a "classic" response to an allergy causing food and results in symptoms such as hives, swelling, and itching (Sampson, 2003).
In the United States, it is estimated that approximately 5.9 million children and approximately 9 million adults have a food allergy (Gupta et al., 2011). Food allergies are most common early in life, impacting children under the age of three years most frequently. These adverse reactions to food may be caused by IgE-mediated immune mechanisms (Sampson, 2003). Most food allergies are IgE-mediated (Beyer & Teuber, 2004).
When an individual with a food allergy experiences an allergic reaction to food, the reaction manifests in a two step process which includes the first exposure and then all subsequent exposures. Generally few symptoms occur the first time that an individual with a food allergy is exposed to the food and the symptoms are generally mild. Once a person has been exposed to the allergen, however, subsequent exposures regardless of how small can result in significant symptoms (Sampson, 1999). During the initial exposure, the immune system creates an antibody, immunoglobulin E (IgE) to the allergen. An antibody is a protein molecule that is created by the immune system to identify and eradicate toxins, bacteria, and viruses (National Institute of Allergies and Infectious Diseases, 2011). The IgE antibodies circulate throughout the blood and attach to mast cells and basophils, which have receptors that are specifically for IgE antibodies. Mast cells are located in body tissue and basophils are located in the blood stream. The second time and each subsequent time that an individual with a food allergy is exposed to the food, the allergen binds to the IgE antibodies that are attached to the mast cells and basophils. When two IgE antibodies bind to the allergen, the mast cell or basophil is broken down, in a process called degranulation (Sampson, 1999). This causes the cells to release large quantities of chemicals (National Institute of Allergy and Infectious Diseases, 2010). These chemicals are referred to as mediators, meaning that they are excreted by cells due to the allergen interacting with the antibody. The chemicals that are released include histamine, slow-reacting substance-A (SRS-A), leukotrienes, typtase, and prostaglandins. The release of the chemicals causes the symptoms of an allergic reaction which include an increase in mucous, airway edema (swelling), bronchospasm (which causes breathing difficulties), extremely low blood pressure, bronchial smooth muscle tone and vascular permeability (Ryder & Waldmann, 2004).
Types of Foods
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Despite an abundance and variety of food available in the United States, only a small number of foods typically evoke an allergic reaction in individuals with food allergies. Approximately 90% of the food allergy reactions that occur in young children come from milk, eggs, peanuts, soy, and wheat. Approximately 85% of the food allergy reactions that occur in adolescents and adults come from peanuts, fish, shellfish, and tree nuts (Sampson, 1999). Most food allergies present in children are outgrown; however fish, tree nuts, and peanuts tend to be allergens that remain with a patient throughout life (Bannon, 2004). Outgrowing a food allergy is a complicated process, unique to the individual patient and the specific food that causes a reaction. It is largely unknown as to why certain foods allergies are generally outgrown and others are not. In allergies that are outgrown, antibodies that are allergen specific tend to decrease over time whereas in allergies that are not outgrown, the antibody levels tend to remain consistent (Wood, 2003).
Most of the foods available in the United States undergo processing. Means of processing include harvesting, cooking, roasting, freezing, storing, and heating. These different processes have an impact on food proteins. Processing that involves heating often reduces the allergenic effects of the food, but in some cases, heat can also increase the allergenic effects of the food (Beyer & Teuber, 2004). For example, heating reduces the allergenic effects of ovalbumin, a protein and ovomucoid, a mucoprotein that are found in egg whites (Allen, Campbell, & Kemp, 2007). In contrast, the high temperatures that are used to dry roast peanuts increase the allergenic effects of the peanut (Sampson, 2004).
Factors Associated with Food Allergies
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While the causes of food allergies are not entirely understood, genetic and environmental factors are thought to contribute to the risk of developing an allergy to food. Individuals with a parent or sibling who has a food allergy are significantly more likely to develop a food allergy in comparison to individuals without a family history (Lack, 2008). Environmental factors that may increase food allergies include methods of food processing, manufacturing, and preparation (Allen & Koplin, 2012). Medical conditions that are typically co morbid with food allergies include asthma, eczema, respiratory allergies, and atopic dermatitis (Boyce et al., 2010).
Diagnosis of food allergies involves gathering an extensive amount of information regarding the patient's medical history and interactions with the food that is thought to be the cause of the reaction. Information relevant to the diagnosis of a food allergy typically includes: the specific food and the amount that was eaten, the length of time between the consumption of the food and the development of the alleged reactive symptoms, the history of reactive symptoms when the particular food was consumed in the past, the last time the reaction occurred and any other factors that may have contributed to the reaction. Often, it is helpful if the patient maintains a food diary that includes information about foods that were consumed and any reactions that occurred. At times, elimination diets are used in the diagnosis of food allergies. An elimination diet requires the patients to completely avoid the food that is thought to be provoking the symptoms. Depending on the patient and the allergen, an elimination diet can be difficult to maintain (Sampson, 2003).
In the diagnosis of food allergies, the double-blind, placebo controlled food challenge is recognized as the gold standard. With this procedure, a patient who is suspected to have a food allergy eliminates all foods that are suspected to be allergens for two to eight weeks prior to the oral food challenge. When the food challenge occurs, the patient should not be on any medication that could reduce symptoms related to an allergic reaction. The procedure works best when it is conducted in a double-blind trial to eliminate the risk of bias on behalf of the patient or the doctor. It is always conducted in a medical setting so that reactions to foods can be documented and treated appropriately. When the food is introduced in the oral food challenge, a very small amount of the food is given to the patient, much less than would be needed to provoke a reaction. The patient is monitored for symptoms of a reaction and the amount of the food given gradually increases until it is comparable to the amount of food that would be appropriate for one serving (Boyce et al., 2010).
Currently, no treatment for food allergies is available. Those with food allergies are advised to treat the symptoms that are caused by the allergen or to avoid the allergen completely (Boyce et al., 2010). The method of avoiding the foods can cause problems such as nutritional deficiencies and difficulty identifying allergens in prepackaged foods or foods prepared in a restaurant. For children, it can be difficult to identify and avoid allergens in school settings (Sicherer, 2011). Recent studies, however, suggest that strict avoidance of allergens may not be necessary. Some children with food allergies are able to tolerate the food when it has been heated (Kim & Sicherer, 2010).
Averse reactions to foods can manifest in various ways and symptoms generally involve the respiratory system, the skin, and the gastrointestinal tract. Symptoms vary depending on the individual. When an allergen is ingested, a common reaction is to experience sensations such as swelling and itching of the lips, mouth, and throat. As the allergen travels through the gastrointestinal tract, additional common symptoms include diarrhea, nausea, and vomiting. Common skin reactions to allergens include itching, redness of the skin, hives, and eczema. For individuals who have asthma, having an allergic reaction to food can also trigger an asthma attack (Sampson, 1999). These symptoms are caused by the chemicals released in the cells during an allergic reaction. Common chemicals that are released include histamine and leukotrienes (Ryder & Waldmann, 2004). Histamine typically causes sneezing, watery eyes, itching, hives, and a runny nose (Thurmond, Gelfand, & Dunford, 2008). Leukotrienes are responsible for bronchoconstriction, smooth muscle hypertrophy, and an increase of mucous production and secretion (Scow, Luttermoser, & Dickerson, 2007).
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Anaphylaxis is a rare but serious allergic reaction that can be life threatening. Anaphylaxis occurs in the same way that allergic reactions to food occur, but it is a more severe response. It typically occurs very soon after an individual comes in contact with an allergen. The most common and serious symptoms associated with anaphylaxis include breathing difficulties and loss of consciousness (Asthma and Allergy Foundation of America, 2005).
When anaphylaxis occurs as a result of exposure to an allergen, epinephrine is used to manage it. Epinephrine is considered the first choice in treating anaphylaxis and there are no known contraindications. Typically, in patients with serious food allergies, epinephrine is prescribed as an emergency supportive therapy. For such purposes, the epinephrine is prescribed in the form of a single dose, self administered injection, some patients require a second dose of epinephrine (Simmons, 2009). Epinephrine increases blood pressure, reverses dilation of blood vessels, decreases swelling under the skin, and reduces hives. (Ellis & Day, 2003).
Impact on Quality of Life
Due to the fact that there are no available treatments for food allergies, an individual with a food allergy must avoid foods that may contain the allergen and manage the symptoms of a reaction in the event that exposure occurs. Avoidance of the allergen in daily life requires careful reading of labels on commercially packaged food, avoidance of cross-contamination (such as food being prepared on the same surface as an allergen), and reducing possible exposure in a variety of settings including school, work, and social environments (Sicherer, Noone, & Munoz-Furlong, 2001).
Bollinger et al. (2006) conducted a study examining how food allergies impact the daily living functions of children with food allergies and their parents. Results indicated that in families that include a child with a food allergy, the allergy significantly impacts almost all areas of daily functioning. Meal preparations and socialization were the most significantly impacted areas of life. Many of the parents expressed concerns about leaving their children with others, allowing them to go to a social event independently, or allowing others to prepare food for their children in fear of an accidental exposure to an allergen.
While research provides a strong understanding of how food allergies occur in the body, many questions still remain. At this time, treatments are limited for IgE mediated food allergies and are focused mainly on allergen avoidance, which can be difficult and impact quality of life. Additionally, there is limited knowledge about the cause of food allergies and what can be done to prevent them.