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. late-phase response after the chemical mediators of the acute response subside, late-phase responses can often occur 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. diagnosis skin testing on the arm is a common way for detecting an allergy, but it is not as effective as other tests. patch test diagnosis is usually based on a medical history, elimination diet, skin prick test, blood tests for food-specific ige antibodies, or oral food challenge. for skin-prick tests, a tiny board with protruding needles is used. the allergens are placed either on the board or directly on the skin. the board is then placed on the skin, to puncture the skin and for the allergens to enter the body. if a hive appears, the person is considered positive for the allergy. this test only works for ige antibodies. allergic reactions caused by other antibodies cannot be detected through skin-prick tests. skin-prick testing is easy to do and results are available in minutes. different allergists may use different devices for testing. some use a "bifurcated needle", which looks like a fork with two prongs. others use a "multitest", 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 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. patch testing is used to determine if a specific substance causes allergic inflammation of the skin. it tests for delayed food reactions. blood testing is another way to test for allergies; however, it poses the same disadvantage and only detects ige allergens and does not work for every possible allergen. radioallergosorbent testing (rast) is used to detect ige antibodies present to a certain allergen. the score taken from the rast is compared to predictive values, taken from a specific type of rast. if the score is higher than the predictive values, a great chance the allergy is present in the person exists. one advantage of this test is that it can test many allergens at one time. a cap-rast has greater specificity than rast; it can show the amount of ige present to each allergen. researchers have been able to determine "predictive values" for certain foods, which can be compared to the rast results. if a person's rast score is higher than the predictive value for that food, over a 95% chance exists that patients will have an allergic reaction (limited to rash and anaphylaxis reactions) if they ingest that food. currently, predictive values are available for 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 and the food allergy profile are considered unproven methods, the use of which is not advised. food challenges test for allergens other than those caused by ige allergens. the allergen is given to the person in the form of a pill, so the person can ingest the allergen directly. the person is watched for signs and symptoms. the problem with food challenges is that they must be performed in the hospital under careful watch, due to the possibility of anaphylaxis. food challenges, especially double-blind, placebo-controlled food challenges, are the gold standard for diagnosis of food allergies, including most non-ige-mediated reactions, but is rarely done. 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 recommended 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. additional diagnostic tools for evaluation of eosinophilic or non-ige mediated reactions include endoscopy, colonoscopy, and biopsy. differential diagnosis important differential diagnoses are: lactose intolerance generally develops later in life, but can present in young patients in severe cases. it is not an immune reaction and is due to an enzyme deficiency (lactase). it is more common in many non-western people. celiac disease. while it is caused by a permanent intolerance to gluten (present in wheat, rye, barley and oats), is not an allergy nor simply an intolerance, but a chronic, multiple-organ autoimmune disorder primarily affecting the small intestine. irritable bowel syndrome c1 esterase inhibitor deficiency (hereditary angioedema), a rare disease, generally causes attacks of angioedema, but can present solely with abdominal pain and occasional diarrhea, and thus may be confused with allergy-triggered angioedema. prevention breastfeeding for more than four months may prevent atopic dermatitis, cow's milk allergy, and wheezing in early childhood. early exposure to potential allergens may be protective. specifically, early exposure to eggs and peanuts reduces the risk of allergies to these. guidelines suggest introducing peanuts as early as 4–6 months and include precautionary measures for high-risk infants. the former guidelines, advising delaying the introduction of peanuts, are now thought to have contributed to the increase in peanut allergy seen recently. 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. in some cases, hypersensitive reactions can be triggered by exposures to allergens through skin contact, inhalation, kissing, participation in sports, blood transfusions, cosmetics, and alcohol. early introduction of peanut and egg alongside other solids, or by one year of age, may help prevent development of food allergy. introduction of these allergenic foods within the first year of life appears to be safe. a window of opportunity for the introduction of different food allergens may exist, such as egg introduction ahead of peanut. inhalation exposure allergic reactions to airborne particles or vapors of known food allergens have been reported as an occupational consequence of people working in the food industry, but can also take place in home situations, restaurants, or confined spaces such as airplanes. according to two reviews, respiratory symptoms are common, but in some cases there has been progression to anaphylaxis. the most frequent reported cases of reactions by inhalation of allergenic foods were due to peanut, seafood, legumes, tree nut, and cow's milk. steam rising from cooking of lentils, green beans, chickpeas and fish has been well documented as triggering reactions, including anaphylactic reactions. one review mentioned case study examples of allergic responses to inhalation of other foods, including examples in which oral consumption of the food is tolerated. treatment the mainstay of treatment for food allergy is total avoidance of the foods identified as allergens. an allergen can enter the body by consuming a portion of 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 problematic food. total avoidance is complicated because the declaration of the presence of trace amounts of allergens in foods is not mandatory (see regulation of labelling). if the food is accidentally ingested and a systemic reaction (anaphylaxis) occurs, then epinephrine should be used. 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. epinephrine epinephrine autoinjectors are portable single-dose epinephrine-dispensing devices used to treat anaphylaxis. epinephrine (adrenaline) is the first-line treatment for severe allergic reactions (anaphylaxis). if administered in a timely manner, epinephrine can reverse its effects. epinephrine relieves airway swelling and obstruction, and improves blood circulation; blood vessels are tightened and heart rate is increased, improving circulation to body organs. epinephrine is available by prescription in an autoinjector. antihistamines 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. steroids glucocorticoid steroids are used to calm down the immune system cells that are attacked by the chemicals released during an allergic reaction. this 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 is the delay in reducing inflammation. steroids can also be taken orally or through injection, by which every part of the body can be reached and treated, but a long time is usually needed for these to take effect. epidemiology food allergies affect up to 10% of the worldwide population, and they are currently more prevalent in children (~8%) than adults (~5) in western nations. in several industrialized countries, food allergies affect up to 10% of children. children are most commonly allergic to cow's milk, chicken eggs, peanuts, and tree nuts. while studies on adults with food allergy are not as abundant, surveys suggest that the most common food allergens for adults include fish, shellfish, peanuts, and tree nuts. food allergies have become increasingly prevalent in industrialized/westernized nations over the last 2-3 decades. an estimated 15 million people currently have food allergies in the united states. in 1997, 0.4% children in the united states were reported to have peanut allergy, yet this number markedly rose to 1.4% in 2008. in australia, hospital admission rates for food-induced anaphylaxis increased by an average of 13.2% from 1994-2005. one possible explanation for the rise in food allergy is the "old friends" hypothesis, which