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|Classification and external resources|
|Classification and external resources|
The protein in the food is the most common allergic component. These kinds of allergies occur when the body's immune system mistakenly identifies a protein as harmful. Some proteins or fragments of proteins are resistant to digestion and those that are not broken down in the digestive process are tagged by the Immunoglobulin E (IgE). These tags fool the immune system into thinking that the protein is an invader. The immune system, thinking the organism (the individual) is under attack, sends white blood cells to attack, and that triggers an allergic reaction. These reactions can range from mild to severe. Allergic responses include dermatitis, gastrointestinal and respiratory distress, including such life-threatening anaphylactic responses as biphasic anaphylaxis and vasodilation; these require immediate emergency intervention. Individuals with protein allergies commonly avoid contact with the problematic protein. Some medications may prevent, minimize or treat protein allergy reactions. There is no cure.
Treatment consists of either immunotherapy (desensitisation) or avoidance, in which the allergic person avoids all forms of contact with the food to which they are allergic. Areas of research include anti-IgE antibody (omalizumab, or Xolair) and specific oral tolerance induction (SOTI), which have shown some promise for treatment of certain food allergies. People diagnosed with a food allergy may carry an injectable form of epinephrine such as an EpiPen, or wear some form of medical alert jewelry, or develop an emergency action plan, in accordance with their doctor.
The scope of the problem, particularly for young people, is a significant public health issue.
Food allergy is thought to develop more easily in patients with the atopic syndrome, a very common combination of diseases: allergic rhinitis and conjunctivitis, eczema and asthma. The syndrome has a strong inherited component; a family history of allergic diseases can be indicative of the atopic syndrome.
Conditions caused by food allergies are classified into 3 groups according to the mechanism of the allergic response:
1. IgE-mediated (classic):
2. IgE and/or non-IgE-mediated:
3. Non-IgE mediated:
Classic immunoglobulin-E (IgE)-mediated food allergies are classified as type-I immediate Hypersensitivity reaction. These allergic reactions have an acute onset (from seconds to one hour) and may include:
Symptoms of allergies vary from person to person. The amount of food needed to trigger a reaction also varies from person to person.
Serious danger regarding allergies can begin when the respiratory tract or blood circulation is affected. The latter can be indicated through wheezing and cyanosis. Poor blood circulation leads to a weak pulse, pale skin, and fainting.
A severe case of an allergic reaction, caused by symptoms affecting the respiratory tract and blood circulation, is called anaphylaxis. When symptoms are related to a drop in blood pressure, the person is said to be in anaphylactic shock. Anaphylaxis occurs when IgE Antibodies are involved, and areas of the body that are not in direct contact with the food become affected and show symptoms. This occurs because no nutrients are circulated throughout the body, causing the widening of blood vessels. This vasodilation causes blood pressure to decrease, which leads to the loss of consciousness. Those with asthma or an allergy to peanuts, tree nuts, or seafood are at greater risk for anaphylaxis.
One of the most common food allergies is a sensitivity to peanuts, a member of the bean family. Peanut allergies may be severe, but children with peanut allergies sometimes outgrow them. Tree nuts, including pecans, pistachios, pine nuts, coconuts, and walnuts, are also common allergens. Sufferers may be sensitive to one particular tree nut or to many different tree nuts. Also seeds, including sesame seeds and poppy seeds, contain oils where protein is present, which may elicit an allergic reaction.
Milk, from cows, goats or sheep, is another common food allergen, and many sufferers are also unable to tolerate dairy products such as cheese. A small portion of children with a milk allergy, roughly ten percent, will have a reaction to beef. Beef contains a small amount of protein that is present in cow's milk.
Other foods containing allergenic proteins include soy, wheat, fish, shellfish, fruits, vegetables, maize, spices, synthetic and natural colors, and chemical additives.
Although sensitivity levels vary by country, the most common food allergies are allergies to milk, eggs, peanuts, tree nuts, seafood, shellfish, soy and wheat. [clarification needed] These are often referred to as "the big eight." Allergies to seeds — especially sesame — seem to be increasing in many countries. An example of allergies more common to a particular region is the surfeit of rice allergies in East Asia where rice forms a large part of the diet.
Some children who are allergic to cow's milk protein also show a cross sensitivity to soy-based products. There are infant formulas in which the milk and soy proteins are degraded so when taken by an infant, their immune system does not recognize the allergen and they can safely consume the product. Hypoallergenic infant formulas can be based on hydrolyzed proteins, which are proteins partially predigested in a less antigenic form. Other formulas, based on free amino acids, are the least antigenic and provide complete nutrition support in severe forms of milk allergy.
People with latex allergy often also develop allergies to bananas, kiwifruit, avocados, and some other foods.
Allergic reactions are hyperactive responses of the immune system to generally innocuous substances. When immune cells encounter the allergenic protein, IgE antibodies are produced; this is similar to the immune system's reaction to foreign pathogens. The IgE antibodies identify the allergenic proteins as harmful and initiate the allergic reaction. The harmful proteins are those that do not break down due to the strong bonds of the protein. IgE antibodies bind to a receptor on the surface of the protein, creating a tag, just as a virus or parasite becomes tagged. It is not entirely clear why some proteins do not denature and subsequently trigger allergic reactions and hypersensitivity while others do not.
Hypersensitivities are categorized according to the parts of the immune system that are attacked and the amount of time it takes for the response to occur. There are four types of Hypersensitivity reaction: Type 1, Immediate IgE-mediated, Type 2, Cytotoxic, Type 3, Immune complex-mediated, and Type 4, Delayed cell-mediated. The pathophysiology of allergic responses can be divided into two phases. The first is an acute response that occurs immediately after exposure to an allergen. This phase can either subside or progress into a "late phase reaction" which can substantially prolong the symptoms of a response, and result in tissue damage.
Many food allergies are caused by hypersensitivities to particular proteins in different foods. Proteins have unique properties that allow them to become allergens, such as stabilizing forces in the tertiary and quaternary structure which prevent degradation during digestion. Many theoretically allergenic proteins cannot survive the destructive environment of the digestive tract and thus don't trigger hypersensitive reactions.
In the early stages of allergy, a type I hypersensitivity reaction against an allergen, encountered for the first time, causes a response in a type of immune cell called a TH2 lymphocyte, which belongs to a subset of T cells that produce a cytokine called interleukin-4 (IL-4). These TH2 cells interact with other lymphocytes called B cells, whose role is the production of antibodies. Coupled with signals provided by IL-4, this interaction stimulates the B cell to begin production of a large amount of a particular type of antibody known as IgE. Secreted IgE circulates in the blood and binds to an IgE-specific receptor (a kind of Fc receptor called FcεRI) on the surface of other kinds of immune cells called mast cells and basophils, which are both involved in the acute inflammatory response. The IgE-coated cells, at this stage are sensitized to the allergen.
If later exposure to the same allergen occurs, the allergen can bind to the IgE molecules held on the surface of the mast cells or basophils. Cross-linking of the IgE and Fc receptors occurs when more than one IgE-receptor complex interacts with the same allergenic molecule, and activates the sensitized cell. Activated mast cells and basophils undergo a process called degranulation, during which they release histamine and other inflammatory chemical mediators (cytokines, interleukins, leukotrienes, and prostaglandins) from their granules into the surrounding tissue causing several systemic effects, such as vasodilation, mucous secretion, nerve stimulation and smooth muscle contraction. This results in rhinorrhea, itchiness, dyspnea, and anaphylaxis. Depending on the individual, the allergen, and the mode of introduction, the symptoms can be system-wide (classical anaphylaxis), or localized to particular body systems; asthma is localized to the respiratory system and eczema is localized to the dermis.
After the chemical mediators of the acute response subside, late phase responses can often occur. This is due to the migration of other leukocytes such as neutrophils, lymphocytes, eosinophils, and macrophages to the initial site. The reaction is usually seen 2–24 hours after the original reaction. Cytokines from mast cells may also play a role in the persistence of long-term effects. Late phase responses seen in asthma are slightly different from those seen in other allergic responses, although they are still caused by release of mediators from eosinophils, and are still dependent on activity of TH2 cells.
There are four common types of allergy testing: skin prick test, patch test, blood test, and food challenges. An allergist can perform these tests, and they can also go into further depth depending on the results.
Skin prick testing is easy to do and results are available in minutes. Different allergists may use different devices for skin prick testing. Some use a "bifurcated needle", which looks like a fork with 2 prongs. Others use a "multi-test", which may look like a small board with several pins sticking out of it. In these tests, a tiny amount of the suspected allergen is put onto the skin or into a testing device, and the device is placed on the skin to prick, or break through, the top layer of skin. This puts a small amount of the allergen under the skin. A hive will form at any spot where the person is allergic. This test generally yields a positive or negative result. It is good for quickly learning if a person is allergic to a particular food or not, because it detects allergic antibodies known as IgE. Skin tests cannot predict if a reaction would occur or what kind of reaction might occur if a person ingests that particular allergen. They can however confirm an allergy in light of a patient's history of reactions to a particular food. Non-IgE mediated allergies cannot be detected by this method.
Blood tests are another useful diagnostic tool for evaluating IgE-mediated food allergies. For example, the RAST (RadioAllergoSorbent Test) detects the presence of IgE antibodies to a particular allergen. A CAP-RAST test is a specific type of RAST test with greater specificity: it can show the amount of IgE present to each allergen. Researchers have been able to determine "predictive values" for certain foods. These predictive values can be compared to the RAST blood test results. If a persons RAST score is higher than the predictive value for that food, then there is over a 95% chance the person will have an allergic reaction (limited to rash and anaphylaxis reactions) if they ingest that food. Currently, predictive values are available for the following foods: milk, egg, peanut, fish, soy, and wheat. Blood tests allow for hundreds of allergens to be screened from a single sample, and cover food allergies as well as inhalants. However, non-IgE mediated allergies cannot be detected by this method. Other widely promoted tests such as the antigen leukocyte cellular antibody test (ALCAT) and the Food Allergy Profile are considered unproven methods, the use of which is not advised.
Food challenges, especially double-blind placebo-controlled food challenges (DBPCFC), are the gold standard for diagnosis of food allergies, including most non-IgE mediated reactions. Blind food challenges involve packaging the suspected allergen into a capsule, giving it to the patient, and observing the patient for signs or symptoms of an allergic reaction.
The best method for diagnosing food allergy is to be assessed by an allergist. The allergist will review the patient's history and the symptoms or reactions that have been noted after food ingestion. If the allergist feels the symptoms or reactions are consistent with food allergy, he/she will perform allergy tests.
Important differential diagnoses are:
Nobel Laureate Charles Richet discovered that proteins injected into animals caused them to develop sensitization to those proteins. Subsequent exposure to those proteins would result in anaphylaxis – a term he coined. The detailed mechanism of such allergy development is described in the Acute response section.
Today, it is common to induce food allergy in laboratory mice by injecting them with food proteins along with alum as an adjuvant.
Multiple routes of food protein administration have been shown to cause food allergies in humans.
Stomach acids degrade food proteins. Stomach acid reducing medications such as proton pump inhibitors (PPI), affect protein degradation. When stomach acid is reduced, intact food proteins are absorbed into the blood stream resulting in the development of food allergies.
Antibiotic use increasing the likelihood of food allergies was supported by a case-control study examining 7,533 children that was presented at the 2013 annual meeting of the American Academy of Allergy, Asthma & Immunology.
According to a report issued by the American Academy of Pediatrics:
There is evidence that breastfeeding for at least 4 months, compared with feeding infants formula made with intact cow milk protein, prevents or delays the occurrence of atopic dermatitis, cow milk allergy, and wheezing in early childhood.
In order to avoid an allergic reaction, a strict diet can be followed. It is difficult to determine the amount of allergenic food required to elicit a reaction, so complete avoidance should be attempted unless otherwise suggested by a qualified medical professional. In some cases, hypersensitive reactions can be triggered by exposures to allergens though skin contact, inhalation, kissing, participation in sports, blood transfusions, cosmetics, and alcohol.
When avoiding certain foods in order to lessen the risk of reaction, it can be difficult to maintain the proper amounts of nutrients. Some allergens are also common sources of vitamins and minerals, as well as macronutrients such as fat and protein; healthcare providers will often suggest alternate food sources of essential vitamins and minerals which are less allergenic.
The mainstay of treatment for food allergy is total avoidance of the foods that have been identified as allergens. An allergen can enter the body by consuming a food containing the allergen, and can also be ingested by touching any surfaces that may have come into contact with the allergen, then touching the eyes or nose. For people who are extremely sensitive, avoidance includes avoiding touching or inhaling the problematic food.
If the food is accidentally ingested and a systemic reaction (anaphylaxis) occurs, then epinephrine should be used. It is possible that a second dose of epinephrine may be required for severe reactions. The person should then be transported to the emergency room, where additional treatment can be given. Other treatments include: antihistamines, and steroids.In a severe case, neither steroids nor anihistamines are life saving and its use should be limited to adjuvant therapy following epinephrine administration.
Epinephrine is another name for the hormone adrenaline, which is produced naturally in the body. An epinephrine shot is the first-line treatment for severe allergic reactions (known as anaphylaxis). If administered in a timely manner, epinephrine can reverse the effects of anaphylaxis.
Epinephrine relieves airway swelling and obstruction and improves blood circulation; blood vessels are tightened and heart rate is increased, improving circulation to bodily organs. Epinephrine is available by prescription in an auto-injector which is used in the treatment of anaphylaxis.
Antihistamines can alleviate some of the milder symptoms of an allergic reaction, but do not treat all symptoms of anaphylaxis. Antihistamines block the action of histamine, which causes blood vessels to dilate and become leaky to plasma proteins. Histamine also causes itchiness by acting on sensory nerve terminals. The most common antihistamine given for food allergies is diphenhydramine, also known as Benedryl.
Steroids are used to calm down the immune system cells that are attacked by the chemicals released during an allergic reaction. This form of treatment in the form of a nasal spray should not be used to treat anaphylaxis, for it only relieves symptoms in the area in which the steroid is in contact. Another reason steroids should not be used to treat anaphylaxis is due to the long amount of time it takes to reduce inflammation and start to work. Steroids can also be taken orally or through injection. By taking a steroid in these manners, every part of the body can be reached and treated, but a long time is usually needed for these to take effect.
For food allergy, desensitization through sublingual, oral or epicutaneous immunotherapy is possible for a subset of people, but not all. While not a cure, this program enables food allergic children and adults to consume foods that they were allergic to previously, without any allergic reaction.
Desensitization approaches for food allergy are generally at the research stage. They include:
As of early 2012, phase II clinical trials for human efficacy have gotten underway for a formula called FAHF-2 (food allergy herbal formula 2). This formula is based on a long-used Traditional Chinese Medicine formula for parasite infection. In early clinical trials, it has been found to completely block anaphylaxis in mouse models. 
The most common food allergens include peanuts, milk, eggs, tree nuts, fish, shellfish, soy, and wheat — these foods account for about 90% of all allergic reactions. The most common food allergies in adults are shellfish, peanuts, tree nuts, fish, and egg. The most common food allergies in children are milk, eggs, peanuts, and tree nuts.
Six to eight percent of children under the age of three have food allergies and nearly four percent of adults have food allergies.
For reasons that are not entirely understood, the diagnosis of food allergies has apparently become more common in Western nations in recent times. In the United States, food allergy affects as many as 5% of infants less than three years of age and 3% to 4% of adults. There is a similar prevalence in Canada.
Seventy-five percent of children who have allergies to milk protein are able to tolerate baked-in milk products, i.e., muffins, cookies, cake and hydrolysed formulas.
About 50% of children with allergies to milk, egg, soy, peanuts, tree nuts and wheat will outgrow their allergy by the age of 6. Those that are still allergic by the age of 12 or so have less than an 8% chance of outgrowing the allergy.
Peanut and tree nut allergies are less likely to be outgrown, although evidence now shows that about 20% of those with peanut allergies and 9% of those with tree nut allergies will outgrow them.
Meat allergy is extremely rare in the general population, but a geographic cluster of people allergic to meat has been observed in Sydney, Australia. There appears to be a possible association between localised reaction to tick bite and the development of meat allergy.
Fruit allergies exist, such as to apples, peaches, pears, jackfruit, strawberries, etc.. This is suspected to be associated with ragweed pollen allergy but could be due to other factors.
Corn allergy may also be prevalent in many populations, although it may be difficult to recognize in areas such as the United States and Canada where corn derivatives are common in the food supply.
In response to the risk that certain foods pose to those with food allergies, some countries have responded by instituting labeling laws that require food products to clearly inform consumers if their products contain major allergens or by-products of major allergens. Some countries also require companies to warn customers when food has been prepared around certain allergens that have been known to cause severe reactions.
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