What are food allergies?

Quick Answer
An abnormal response by the immune system to some foods, causing mild to severe symptoms that may become life threatening.
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Causes and Symptoms

Allergic reactions occur when the immune system is stimulated to protect the body from foreign organisms known as allergens. Allergens, usually proteins, are perceived by the body as potentially harmful. When the immune system is activated, two types of white blood cells respond: phagocytes and lymphocytes. Phagocytes destroy bacteria, viruses, and parasites. Lymphocytes destroy other types of harmful organisms, and it is these white blood cells that respond when a food is perceived as harmful. Therefore, when an allergen is encountered, white blood cells respond and attach an immunoglobulin (Ig) antibody to it. Five different antibodies can be activated, and each has a different responsibility. Immunoglobulin E (IgE) is the antibody that responds during a food allergy reaction. Marking an allergen with an Ig antibody distinguishes it from healthy tissue and cells. This in turn allows white blood cells to release a chemical spray, usually histamine, which targets and destroys only the harmful allergen.

Food is essential for life and good health, and the exact reason why a food substance is identified as harmful is still inconclusive. Genetics provide the strongest link to food allergy incidence. Research studies show that a family history of allergies increases the chance of developing all allergies, including those to food. If both parents have an allergy, there is up to a 75 percent chance that their children will also develop an allergy. Even those who have no family history of allergies can still develop them, however, indicating that other factors play a role. Many theories have been proposed. The hygiene hypothesis suggests that the Western world’s habit of cleanliness causes the immune system to become bored and attack itself. Introducing foods at too early of an age may overstimulate an immature gastrointestinal tract and trigger an allergic reaction. The leaky gut theory, in which an unhealthy gastrointestinal tract leaks allergens into the bloodstream, is also believed to promote allergies. Many foods also contain proteins, and it is thought the body confuses food proteins with an allergenic protein from another source. Frequent use of antacids, food additives, and vaccines, genetic manipulations of food crops, and exposure to environmental toxins are also linked to an abnormal immune system response to foods.

More than two hundred foods, some food additives, and foods naturally high in histamine have been reported to cause adverse food reactions. However, not all adverse food reactions are diagnosed as a food allergy. According to the US Centers for Disease Control and Prevention, an estimated 5.6 percent of children reported having a food allergy in 2012. The majority of food allergies are to cow’s milk, eggs, fish, peanuts, shellfish, soybeans, tree nuts, and wheat. Frequently, food allergy symptoms mimic food intolerances, sensitivities, and other medical disorders, making diagnosis difficult. In general, a true food allergy reaction occurs anywhere from minutes to two hours after eating a specific food. Symptoms range from mild to a severe, life-threatening anaphylaxis. Severity of reactions is unpredictable, no matter how mild or severe the previous reaction was. Most symptoms involve the circulatory and respiratory systems, and common symptoms include difficulty breathing, facial swelling, heart arrhythmias, runny or stuffy nose, and fainting. Mild symptoms often affect the skin, causing angioedema, eczema, and hives. The gastrointestinal system may be affected, causing abdominal pain, bloating, diarrhea, metallic taste in the mouth, difficulty swallowing, and vomiting. Children may experience weight loss or poor growth. Malnutrition and eating disorders may also occur over time when one or more food groups must be eliminated from the diet or there is fear of experiencing a reaction.

Many individuals assume, without a medical evaluation, that they have a food allergy. If a food allergy is suspected, then a food allergist, as well as a gastroenterologist when indicated, should be consulted. Medical and family history, blood and skin tests, elimination diet, and food challenge are among the tools used to make a firm diagnosis of food allergies. Blood tests include checking serum IgE antibody concentrations and the radioallergosorbent test (RAST), which tests if a reaction to a specific food allergen occurs. Skin tests involve scratching the skin with food extracts and monitoring for allergic reactions, such as hives, but these tests also have a small percentage of false positives. Therefore, an elimination diet, in which suspected food allergens are eliminated from the diet for two to four weeks, followed by a gradual add-back is often prescribed and symptom changes evaluated.

Treatment and Therapy

Allergic reactions to foods must always be taken seriously since symptoms can escalate from minor to life-threatening in a matter of minutes. Individuals with asthma are at the highest risk for severe food reactions. When a severe food reaction occurs, an injection of epinephrine and emergency medical treatment must be administered as quickly as possible. Mild reactions may require an oral/intravenous antihistamine or steroid medication or topical hydrocortisone ointment. Even when symptoms seem to be over or under control, the individual must still be continually monitored for four to six hours after the initial reaction because symptoms can progress rapidly to life-threatening as a result of a delayed reaction known as a biphasic reaction.

The only way to prevent a food allergy reaction is by strictly avoiding the food. Allergic individuals must be careful at all times about every food that they eat because food allergens can go airborne or be used in the processing or cooking of a food. Reading food ingredient labels (disclosures by food processing manufacturers are required by law) and questioning food preparers about prepared foods are critical prevention steps that anyone living with a food allergy must take. However, accidental exposures can still occur no matter how careful an individual is. Therefore, it is essential that an EpiPen or similar device and a food allergy action plan be carried at all times.

An EpiPen is a device that administers a prescribed dose of injectable epinephrine during severe allergic reactions; quick treatment with it can mean the difference between life and death. Food allergy action plans are also important because they identify the problem and outline a quick and appropriate lifesaving plan of care when an individual is unconscious. A food allergy action plan lists allergenic foods, symptoms, medications and dose prescribed, sequence of steps to follow during an emergency, emergency contact information, and the name and phone number of the treating physician. Wearing medical alert jewelry is also advised.

While adults can develop food allergies as they age, young children frequently outgrow them, although this is not always true for fish, peanut, shellfish, and tree nut allergies. Therefore, it may be recommended that children undergo food challenges periodically to evaluate if they remain allergic to a specific food. Food challenges reintroduce an allergenic food into the diet under strict medical supervision only, since the potential for a severe and fatal allergic reaction is high.

Perspective and Prospects

It has long been recognized that surviving an illness often provides protection against that illness in the future. This connection between immunity and disease was particularly apparent during the ancient plagues of smallpox. Smallpox, an infectious virus, is estimated to have killed three hundred to five hundred million people worldwide in just the twentieth century. Ancient Egyptian mummies show the ravages of this disease, and Greek historians record the decimated population during the Athenian plague (430–426 BCE). Historians have recorded efforts to increase population survival rates, namely the inhaling of crushed smallpox scabs (early immunization technique) and inoculations using pus from smallpox lesions (early vaccination technique) to prevent the disease.

During the eighteenth and nineteenth centuries, many lifesaving advances were made in the fields of science and medicine. The body’s defense mechanisms, the immune system, were explored and many diseases were successfully eradicated over time. Preventative vaccines to protect and increase life span were developed and are now accepted medical practice. However, understanding the mechanism of food allergies has taken much longer. In the twentieth century, Carl Prausnitz-Giles, a bacteriologist-immunologist, was the first to discover that food allergies are intimately tied to the immune system. Scientists Kimishige Ishizaka and Terako Ishizaka discovered that IgE is the principal agent for food allergy reactions.

Research in the area of food allergies has greatly expanded in the twenty-first century, since many countries have experienced an increase in food allergy incidence. In 2008, the United States implemented the Exploratory Investigations in Food Allergy program to focus on research into the origin and epidemiology of food allergies and the discovery of more reliable testing methods and effective treatments. The EuroPrevall Project includes European countries as well as Australia, China, Ghana, India, and New Zealand. The program was initiated in 2005 to improve the quality of life for those who live and struggle with food allergies as well as to discover breakthroughs in clinical research.

Progress is being made in developing more reliable diagnostic methods and treatment options. Promising areas include using pure allergens rather than extracts for skin and blood tests and improved technologies to measure IgE antibodies and the immune response to foods. Also promising are the development of safe injectable and sublingual immunotherapies as preventive treatments, along with anti-IgE medicines that may block allergic reactions.


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