Immunoglobulin E (IgE)-Mediated Food Allergy in Children: Epidemiology, Pathogenesis, Diagnosis, Prevention, and Management

A food allergy is an immunoglobulin E (IgE)-mediated hypersensitive reaction to food, which consists in the appearance of allergic symptoms; it can vary from common urticaria to even fatal anaphylaxis. The prevalence of food allergies has been increasing in the past twenty years and it represents a major public health problem in industrialized countries. The mechanism that leads to food allergies is the lack of immunologic and clinical tolerance to food allergens. The diagnosis of IgE-mediated food allergies is based on the combined use of a detailed medical history, in-vivo, and in-vitro research of specific IgE, the elimination diet, and the double-blind placebo-controlled food challenge. The only currently available treatment for allergies is the strict elimination diet. This type of attitude, which we could define as “passive”, does not overcome the risk of accidental reactions due to involuntary intake of the culprit food. For food allergy management, an “active” approach is urgently needed, such as specific allergen immunotherapy, which is currently under development and only used for research purposes. This article aims to give an updated review of IgE-mediated food allergies in pediatric populations in terms of epidemiology, pathogenesis, prevention, diagnosis, and management.


Introduction
A food allergy is a hypersensitivity response to a specific food antigen. It is classified in immunoglobulin E (IgE) and non-IgE-mediated food allergies on the basis of the time elapsed from the food ingestion to the onset of clinical manifestation, within or later than 2 h, respectively [1].
The aim of this manuscript is to give an updated review of IgE-mediated food allergies in children, in terms of epidemiology, pathogenesis, prevention, diagnosis, and management. As the topic is broad and complex, the review will give an overview of the main concepts of food allergies in children. In particular, chapter 5, entitled "Management" has relevance, primarily for pediatricians and allergists situated in the European community.
The bibliographic research was performed in November 2019 and it was limited to papers published in the last 10 years in English. Other articles have been identified through bibliographies of relevant articles. The relevant articles were identified using the MEDLINE database with the PubMed search engine, using key terms related to food allergies: "children", "IgE-mediated food allergy", "IgE-mediated food hypersensitivity", "epidemiology"," pathogenesis", "prevention", "diagnosis", "treatment", and "immunotherapy". To avoid excluding important studies, the research was not restricted by type of publication or study design. active system, conversely, can take place through the microfold (M) cells, the goblet cells, specialized macrophages that express CX3C chemokines receptors 1 on their surface (CX3CR1), or through the CD103 + dendritic cells in the lamina propria [16,17].
Under normal conditions, these cells induce regulatory T cells (Treg) through the production of IL-10 by macrophages or through the production of transforming growth factor- (TGF-) by dendritic cells [18,19].
The tolerance to food antigens breaks down, in some situations, for example in the event of exposure to certain pathogen-associated molecular patterns (PAMPs) or following epithelial damage, leading to the production of IL-25, IL-33, and thymic stromal lymphopoietin (TSLP). Under these conditions, the induction of Treg cells is altered and switched to antigen specific Th2 cells, which, by producing IL-4, stimulate the B cells to produce immunoglobulin E (IgE) and stimulate mast cell expansion. IL-4 also suppresses the tolerogenic function of Treg and reprograms Treg to produce IL-4 themselves, transforming them from tolerogenic cells to pathogenetic ones [2,20]. Type 2 innate lymphoid cells (ILC2s), which are Th2-like cells without antigenic specificity, produce IL-4 and IL-13, blocking the Treg function [21].
IgEs bind the receptors present on the surface of mast cells. When the patient is exposed again to the same food antigen, it binds specific IgE attached to FcεR on mast cell and basophil cell surfaces with degranulation of those cells that release mediators, such as histamine, generating the symptoms of IgE-mediated food allergy reactions [22].
Understanding the pathogenic mechanism underlying IgE-mediated food allergies allows implementation of those measures aimed to restore clinical and immunologic tolerance.

Diagnosis
An accurate diagnosis of IgE-mediated food allergies is crucial. It is based on the combined use of a detailed medical history, the research of specific IgE via testing in-vivo (skin prick test, SPT) and in-vitro (specific IgE, s-IgE), the elimination diet, and the subsequent oral food challenge (OFC) Figure 1 [23].   A detailed medical history is very important and some questions are fundamental for clarifying the symptoms: (1) "What symptoms appeared?"; (2) "What foods are deemed responsible and how many times have they caused reactions?"; (3) "What was the amount of food that triggered the reaction?"; (4) "Was the food cooked or raw?"; (5) "What was the latency between food intake and the onset of symptoms and how long did the reaction last?"; (6) "Was food taken regularly before causing the reaction?"; (7) "Should other factors be taken into consideration such as exercise, infections, the simultaneous intake of certain drugs like aspirin or other NSAIDs, or alcohol?"; (8) "Were drugs administered to treat the reaction, and if so, to what effect?" [23].
After having recorded the medical history, the in-vivo and in-vitro estimation of specific IgE is recommended. The SPT is an easy, fast, cheap, and sensitive method for the diagnosis an IgE-mediated food allergies, although its diagnostic value is limited compared to the DBPCFC [24].
Many studies [25][26][27][28][29] have tried to identify the real diagnostic value of the SPT, and it has clearly emerged that a negative test excludes a food allergy by 90%, whereas a positive test does not confirm the diagnosis, but instead, a state of sensitization. As the likelihood of being allergic increases as the wheal diameter of the SPT increases, diagnostic decision levels have been defined in different studies for the common allergens, usually with a cut-off at 95%-100% positive predictive value (PPV), and they can increase the specificity of the SPT (Table 1). In vitro tests measuring s-IgE is another way to investigate IgE-mediated food allergies. As with SPT, there is a certain correlation between the specific s-IgE concentration and the possibility of a clinical reaction to the specific food. Various studies have attempted to identify precise diagnostic predictive values. In general, it can be stated that undetectable values of s-IgE are associated to a low risk (10-25%) of reaction to the food, while the risk increases with a rise in the levels of s-IgE. The values also vary, according to the type of allergen, Table 1 [25].
When applying this data in clinical practice, we must bear in mind that the reported predictive values refer to the specific populations studied, which may differ from the populations to which these predictive tests need to be applied. Therefore, a certain amount of caution is always necessary with predictive clinical risks [30].
Determination of specific IgE to a single allergen, known as Component-Resolved-Diagnosis (CRD), is a recent advance in the diagnosis of food allergy. For each individual allergen, various components were identified and given a specific name consisting of the first three letters of the genus, the first letter of the species, and a number that usually reflects the chronological order of identification [31]. For example, 'Cor a 1' is the first allergen described in hazelnut, the scientific name of which is Corylus avellana. So far, for hazelnut, 10 components, named Cor a 1-2-8-9-10-11-12-13-14, TLP (thaumatin like protein) have been identified [32]. The awareness of some of these components, such as Cor a 14, has been associated with a greater risk of serious reactions, as also reported for other allergenic molecules of other substances [33].
However, the clinical repercussions of this information have not yet been defined, and it should also be remembered that not all the molecules present in the allergens are currently available for laboratory testing, as many are still to be discovered.
The Basophil Activation Test (BAT) is another test for food allergies that is moving progressively from the laboratory to a useful tool in clinical practice [34]. BAT can potentially be considered as in-vitro OFC, where basophil cells of the patient are exposed to the culprit food extract in a test tube [35]. The BAT is based on flow cytometry where the expression of activation markers is measured on the surface of allergen-stimulated basophils. BAT has been studied in the diagnosis of a variety of food allergies and its reported sensitivity ranges from 77% to 98%, and specificity from 75% to 100% [36][37][38][39][40][41].
The DBPCFC is the gold standard for the diagnosis of food allergies. It consists of gradually administering the suspect food under medical supervision in order to assess the clinical reaction or state of tolerance to said food. As the test is potentially dangerous, it must be carried out by qualified personnel in a suitable healthcare environment for managing any reactions. It is usually discontinued in the presence of a significant objective reaction or a persistent subjective reaction. In doubtful cases, it is repeated after a few days [31].
In daily clinical practice, the OFC is usually performed in open or single blind. The DBPCFC (in which both the patient and the clinicians do not know whether the "real" test food or a placebo is being eaten) is performed in doubtful cases or for research purposes. If the provocation test is negative, it is good practice to confirm it with an open test in which the patient eats the food in its natural form, as this helps exclude false negatives (which are around 1%-3%) [23,42].
The elimination diet is another very useful tool in the diagnosis of food allergy, especially when there is the onset of chronic skin or gastrointestinal reactions. This is normally used in the initial phase of diagnosis in order to verify the reduction of symptoms after exclusion for a few weeks. Elimination diets must always be carried out short-term and for a specific purpose [43].
The diagnosis of food allergies, with the exception of cases of IgE-mediated anaphylaxis, should, in fact, always be confirmed with OFC. Given that a state of tolerance towards the food to which one is allergic is often acquired over time, it is advisable to periodically (once a year) perform an in-vivo and in-vitro test of the individual allergic state, possibly also with an OFC [23].
Tests with no diagnostic value include the provocation-neutralization test, cytotoxic tests, kinesiology, and electrode dermal tests. In regards to the determination of specific IgG for food, their uselessness has been reported in diagnosing a food allergy or programming an exclusion diet [23]. Therefore, there are several tests to diagnose IgE-mediated food allergies; however, clinical history remains the most important tool to attain an accurate diagnosis. In vivo and in vitro tests can be used, together with clinical history, to reduce the need for OFC. The clinician's task is to interpret the results of all available tests to decide if an OFC is needed to reach the diagnosis.

Prevention
Greater attention is being focused on the research of risk factors and preventive measures for reducing the risk of onset of food allergy. Important advances have been made over the last decades with the defining of three separate hypotheses on how food allergies develop, namely, the dual-allergen exposure hypothesis, the vitamin D hypothesis, and the hygiene hypothesis, which are summarized in Table 2 [44][45][46].

Timing the Introduction of Complementary Foods and the Risk of Developing Food Allergies
An important and very heated discussion exists on the linking between the timing of the introduction of solid foods in infants and the risk of developing food allergies during various periods of life.
In the 1980s and 1990s, early exposure to solid foods (before four months of life) was associated with the development of allergic pathologies, especially eczema [47][48][49].
For this reason, a strategy of allergen avoidance in high-risk newborn infants (with a history of allergies in first-degree relatives) was thus adopted for many years, reducing-as much as possible-the condition of sensitized proteins, both in intrauterine life and during the first months of life (postponed introduction of potentially allergenic foods such as milk, eggs, fish, peanuts for 1-3 years of life) [50].
The avoidance strategy did not prove to be effective, given that in the following decades the incidence of allergic diseases and food allergies continued to increase.
All of this has led to abandoning the strategy of allergenic avoidance and, in fact, has induced several groups to carry out prospective intervention studies according to the hypothesis that the onset of food allergies in the general population, or in children with a high, medium, or low risk of developing food allergies (familiarity and/or atopic dermatitis and/or sensitization to food) could be prevented by the early introduction of common allergenic food [51][52][53][54][55][56][57][58][59].
With the change in evidence, the recommendations suggested by the most important international guidelines have also changed, as summarized in Table 3. General population/at-risk infants (atopic family history, non-severe eczema) • Complementary foods (including allergenic foods) ≥ 6 months.

•
Continue breastfeeding up to 2 years.

High risk infants with severe eczema
• Allergy testing (skin prick tests and/or s-IgE to egg) (and peanuts in countries with high peanut allergy prevalence) should be preliminary performed, followed by a supervised oral challenge in sensitized children.

•
In countries with limited access to allergy tests. • Expertise-only supervised oral challenges to egg (and peanuts in countries with high peanut allergy prevalence) should be performed.

General population
• Complementary foods (including allergenic foods) from around 6 months.

•
Introduction of allergenic foods should not be delayed.

General population
• Complementary foods (including allergenic foods) from around 6 months.

High risk infants with eczema (particularly early-onset or moderate-severe eczema) or food allergy
• Introduction of eggs and peanuts from 4 months.

•
The benefits of allergy testing prior to introducing eggs and peanuts should be balanced against the risk of a delayed introduction. Solid foods should be introduced in the infant's diet in accordance with familial and cultural habits, and when possible, while continuing breastfeeding up to two years of age or beyond [61,66]. Therefore, beginning from the fourth month of life, an infant should try almost all foods, according to his/her taste and acquired neuromotoric development stage, such as the ability to chew, to keep the head up, and to stay sitting down appropriately. In infants with food allergies and/or severe eczema [57] with positive SPT to a specific food, the food should be introduced into his/her diet after an oral food challenge conducted under medical supervision.
Understanding how to prevent the development of food allergies will expand knowledge to maintain tolerance to food antigens.

Management
Many authors consider that the only strategy currently available for the treatment of food allergies is the strict elimination diet. This type of attitude, which we could define as "passive", does not overcome the risk of accidental reactions due to the involuntary intake of the culprit food. [67]. Patients with food allergies, at risk of serious reactions, should always carry a drug kit with an adrenaline autoinjector for immediate self-treatment [23]. For food allergy management, an "active" approach is urgently needed, such as specific allergen immunotherapy, which is currently being developed and only used for research purposes [67,68].
There are different types of immunotherapy, based on the administration route, and namely, oral immunotherapy (OIT) where the allergen is swallowed, sublingual immunotherapy (SLIT) where the allergen is held under the tongue for 2 min and then split or swallowed, and epicutaneous immunotherapy (EPIT), where a patch with a food allergen is applied on the skin [69].
The typical protocol of oral immunotherapy includes an initial escalation phase followed by dose build-up and maintenance phases, Figure 2. Solid foods should be introduced in the infant's diet in accordance with familial and cultural habits, and when possible, while continuing breastfeeding up to two years of age or beyond [61,66]. Therefore, beginning from the fourth month of life, an infant should try almost all foods, according to his/her taste and acquired neuromotoric development stage, such as the ability to chew, to keep the head up, and to stay sitting down appropriately. In infants with food allergies and/or severe eczema [57] with positive SPT to a specific food, the food should be introduced into his/her diet after an oral food challenge conducted under medical supervision.
Understanding how to prevent the development of food allergies will expand knowledge to maintain tolerance to food antigens.

Management
Many authors consider that the only strategy currently available for the treatment of food allergies is the strict elimination diet. This type of attitude, which we could define as "passive", does not overcome the risk of accidental reactions due to the involuntary intake of the culprit food. [67]. Patients with food allergies, at risk of serious reactions, should always carry a drug kit with an adrenaline autoinjector for immediate self-treatment [23]. For food allergy management, an "active" approach is urgently needed, such as specific allergen immunotherapy, which is currently being developed and only used for research purposes [67,68].
There are different types of immunotherapy, based on the administration route, and namely, oral immunotherapy (OIT) where the allergen is swallowed, sublingual immunotherapy (SLIT) where the allergen is held under the tongue for 2 min and then split or swallowed, and epicutaneous immunotherapy (EPIT), where a patch with a food allergen is applied on the skin [69].
The typical protocol of oral immunotherapy includes an initial escalation phase followed by dose build-up and maintenance phases, Figure 2. The initial escalation phase is often carried out on a single day and its purpose is to identify the starting daily dose for home administration.
During the build-up phase, the daily dose is normally increased every 15 days until the maintenance dose is reached. At home, the patients should continue to take the same maintenance dose every day. After some months/one year, a DBPCFC is performed to verify tolerance. The initial escalation phase is often carried out on a single day and its purpose is to identify the starting daily dose for home administration.
During the build-up phase, the daily dose is normally increased every 15 days until the maintenance dose is reached. At home, the patients should continue to take the same maintenance dose every day. After some months/one year, a DBPCFC is performed to verify tolerance.
To assess a sustained unresponsiveness, the daily dose is then stopped for a period of 4 to 12 weeks and reintroduced during a DBPCFC. If no adverse reaction occurs, this state is defined permanent tolerance [70].
The aim of immunotherapy is to reach "a state of tolerance" in which the patient does not show any reaction after ingestion of a normal serving of the culprit food despite a period of absence of exposure. If the tolerance is not reached, we talk about desensitization, which indicates the ability to safely assume the culprit food, but it strictly depends on the daily intake of the same food [71].
A study on egg oral immunotherapy conducted by Jones et al. [72] shows how the tolerance is enhanced with the duration of oral immunotherapy. The sustained unresponsiveness increases from 27.5% after 2 years of oral immunotherapy, up to 50% after 4 years.
While the SLIT induces desensitization (RR = 0.26 (95% CI 0.13-0.64)), it is not as good as OIT [69]. In fact, in a study on SLIT for peanut, conducted by Burks et al. [73], the sustained unresponsiveness was reached in only 11% of the patients.
Eosinophilic esophagitis (EoE) is a long-term side effect of OIT, and its prevalence in subjects undergoing OIT varies from 2.7% [74] to 30% [75]. This wide variability derives from the fact that not all patients who develop gastrointestinal symptoms during OIT undergo a gastroesophageal biopsy; therefore, with the risk of overestimating this side effect.
A recent study [76] showed that OIT-induced EoE can be treated with a slower dosage regimen and a lower maintenance dose.
In SLIT, the systemic reactions are less frequent and milder, similar to a placebo arm (RR 0.98 (0.85-1.14)). The local reactions are frequent (7-40% of patients) and represented by the oral allergic syndrome [69]. To date, there have been no cases of EoE developing during the SLIT for the management of food allergy.
An alternative immunotherapy route, to improve the safety of OIT, is epicutaneous immunotherapy. It consists of a patch, called Viaskin, which is applied on the skin. The allergen protein, adhering to the inside of its surface, is dissolved by the moisture from natural trans-epidermal water loss accumulated under the patch. The permeability of the stratum corneum, increased by the moisture collected under the patch, allows native proteins to concentrate near antigen-presenting immune cells. The upper skin is not vascularized; therefore, the systemic absorption of the allergen is almost eliminated [77].
A study on peanuts demonstrated that patients treated with Viaskin did experience a significant increase in a successfully consumed dose compared to the placebo group (placebo vs. VP100 (Viaskin peanut 100 mcg), p = 0.014; placebo vs. VP250 (Viaskin peanut 250 mcg), p = 0.003); in particular the study showed that younger children experienced a more favorable outcome (p = 0.03; age, 4-11 vs. >11 years) [78].
EPIT seems to be safe and well tolerated. The most frequent reaction is a local skin reaction at the application site [78].
A preliminary study on EPIT with milk failed to demonstrate a statistically significant improvement of the cumulative tolerated dose between the active group and the placebo group [79]. The main characteristics of the three immunotherapy routes are summarized in Table 4.
Another way to improve safety is the use of processed food, especially baked egg and milk. Thermal processing alters allergenicity via denaturation of the epitopes, or by altering susceptibility to digestion. Allergenicity may be further reduced via interaction with wheat proteins, in particular gluten, affecting solubility and bioavailability [80]. In baked-food reactive patients, the OIT with baked products is considered as immunotherapy, whereas in baked-food, tolerant patients, it is instead a marker of a milder, more transient allergy phenotype [81]. In 2017, there was very weak evidence from a systematic review that baked egg and milk OIT could accelerate the acquisition of tolerance [82].
In 2018, a randomized controlled trial conducted on 84 patients with milk allergy demonstrated that a statistically significant higher percentage of patients who consumed baked products reached a tolerance to unheated milk versus patients not consuming baked products [83].
An anti-IgE monoclonal antibody was first proposed as an adjuvant to facilitate OIT by reducing allergic reactions induced by OIT [84].
There are studies on omalizumab and OIT with peanuts, [85,86] egg, [87] milk [88,89] and multiple foods [90]. All these studies demonstrate that omalizumab enables faster achievement of the target maintenance dose and reduces the rate and severity of IgE-mediated reactions during oral immunotherapy.
The use of omalizumab is off label for food allergy immunotherapy and its use is recommended for patients with severe food allergy who failed to be cured by oral immunotherapy [91].
A new therapeutic option, which showed promising results, is the use of probiotics with OIT. Tang et al. [92] has studied the co-administration of Lactobacillus rhamnosus with peanut oral immunotherapy in children with peanut allergies in a double-blind, placebo-controlled trial; 82.1% of patients in the active group compared to 3.6% of patients receiving placebo exhibited permanent tolerance within three weeks after stopping treatment.
A follow-up study four years after discontinuing treatment found that the sustained unresponsiveness was maintained by 7 out of 12 patients in the active group versus 1 out of 15 in the placebo group [93].
In terms of food allergy treatment strategies, it remains unsatisfactory that, at the moment, the only effective treatment is the elimination diet. In addition to immunotherapy, the use of biologics also appears to be promising. However, more and larger clinical trials are needed to clarify the potential of these therapeutic strategies.

Conclusions
In conclusion, more research on strategies for optimization of treatment is needed-such as combining administration routes to improve both efficacy and safety of immunotherapy. Moreover, the use of immunomodulatory agents is being developed and, depending on their results, they could become an important possible treatment for food allergies. Funding: This research received no external funding.

Conflicts of Interest:
The authors declare no conflict of interest.