‘Early Introduction’ of Cow’s Milk for Children with IgE-Mediated Cow’s Milk Protein Allergy: A Review of Current and Emerging Approaches for CMPA Management

IgE-mediated cow’s milk protein allergy (CMPA) is one of the most prevalent food allergies in early childhood. Though the cornerstone of management involves the strict avoidance of milk products while awaiting natural tolerance, research increasingly shows that the rates of resolution are slowing down. Therefore, there is a need to explore alternative pathways to promote tolerance to cow’s milk in pediatric populations. This review aims to combine and appraise the scientific literature regarding the three CMPA management methods: avoidance, the milk ladder, and oral immunotherapy (OIT) and their outcomes in terms of efficacy, safety, and immunological effects. Cow’s milk (CM) avoidance virtually protects against allergic reaction until natural tolerance occurs, with hypoallergenic substitutes available in the market, but accidental ingestion represents the main issue for this strategy. Introduction to baked milk using the milk ladder was designed, with most CMPA patients successfully completing the ladder. Similar to baked milk treatment, many OIT protocols also demonstrated decreased IgE and increased IgG4 levels post protocol, as well as a reduction in wheal size diameter. Though these strategies are shown to be safe and effective in CMPA, future clinical trials should compare the safety and effectiveness of these three management strategies.


Introduction
Food allergy is emerging as a significant public health concern, affecting individuals of all ages, ethnicities, and socio-economic strata [1,2]. The prevalence of food allergies varies slightly by region. In the United States, greater than 10% of the population likely suffers from at least one IgE-mediated food allergy, and in Ireland, 4% of infants are affected by allergies-similar to other European countries [1,2]. Moreover, the prevalence of food allergy has been increasing in recent years [3]. Cow's milk protein allergy (CMPA) is one of the more common food allergies, with a prevalence of 1% in children in Ireland. Internationally, studies using self-reporting to determine the prevalence of CMPA found the prevalence ranged from 1.2-17% [4].
CMPA usually presents before 6 months of age and may resolve spontaneously in childhood [5]. However, research increasingly shows that the rates of resolution are slowing down, meaning that the allergy can persist into adulthood [6]. The rates of CMPA in breastfed children have been shown to be lower than in formula-fed children, at 0.5% [5]. Possible risk factors for developing cow's milk protein allergy include premature birth, birth by caesarean section, maternal food allergy, antibiotic during pregnancy, and the introduction of complementary foods when the child is less than 4 months of age [7,8].
The natural history of most CMA patients is characterized by spontaneous acquisition of tolerance before 3 years of age [16]. It has been found that 56% of children formerly diagnosed as CMA became tolerant at one year of age, and 77% at two years of age [18,19]. Moreover, less than 0.5% of adults are reported to be allergic to CM [20]. Avoidance of the trigger food is considered the first and obvious strategy to implement in CMPA, awaiting natural tolerance. Traditionally, it was thought to be a transient allergy with a high rate of resolution in childhood. However, the resolution rate is not heterogeneous across studies (Table 1 and Figure 1). The increasing persistence of CMPA must be taken into consideration when discussing management and treatment.
trients 2023, 15 Dietary avoidance limits the symptoms of CMPA, and it is also part of the diagnost work up of all patients with suspected CMPA. For patients who experienced life-threa ening events, this approach would be considered the "common sense" approach, but b cause the clinical picture related to CMPA can vary, most patients present with vagu symptoms and unclear clinical history, requiring a trial of CM avoidance to confirm th diagnosis.

Evidence of the Effectiveness of CM Avoidance
Evidence supporting solely CM avoidance is sparse. A systematic analysis performe by the European Academy of Allergy and Clinical Immunology (EAACI) on acute an long-term management of food allergy found only one non-randomized comparison which eliminating cow's milk in patients with a positive radioallergosorbent test (RAST was associated with the remission of symptoms and reduced reactions to allergens ov Dietary avoidance limits the symptoms of CMPA, and it is also part of the diagnostic work up of all patients with suspected CMPA. For patients who experienced life-threatening events, this approach would be considered the "common sense" approach, but because the clinical picture related to CMPA can vary, most patients present with vague symptoms and unclear clinical history, requiring a trial of CM avoidance to confirm the diagnosis. Table 1. Natural history of CMA in different populations and settings (adapted and updated from Gianetti et al. [12]

Evidence of the Effectiveness of CM Avoidance
Evidence supporting solely CM avoidance is sparse. A systematic analysis performed by the European Academy of Allergy and Clinical Immunology (EAACI) on acute and long-term management of food allergy found only one non-randomized comparison in which eliminating cow's milk in patients with a positive radioallergosorbent test (RAST) was associated with the remission of symptoms and reduced reactions to allergens over time [35,36]. However, it should be noted that this study evaluated children who were sensitive to cow's milk and not children specifically with an IgE-mediated CMP allergy. Though dietary avoidance reduces or even eliminates the symptoms, dietary restrictions should keep in consideration the patient's nutritional needs, ideally with the guidance of a dietician.

CM Avoidance: Evidence from Alternative Formula Studies
Three studies exploring substitutes for CM assess the acquired tolerance to CM [37][38][39]. de Boissieu and Dupont found that after consuming AAF for a mean duration of 11.4 ± 7.9 months (range, 3.5-41), 41 of 52 children (78.8%) had achieved tolerance to CM by the end of the observational period, with the median age of CMP tolerance being 20.5 months [37].
The two most recent studies cited here explore the role of the gut microbiome in the development of tolerance to CM [38,39]. Canani et al. compared the occurrence of allergic manifestations and subsequent acquisition of tolerance to CM in children with a median age of 5 months consuming either eHCF with the probiotic Lactobacillus rhamnosus GG (LGG) or eHCF alone. Though the incidence of tolerance increased in both groups, the incidence was significantly higher in the eHCF + LGG group than in the eHCF alone group (p < 0.01 at 12 months) [38]. This study provides evidence that changes in the developing gut microbiome in early life may speed up the acquisition of tolerance to CM. In the most recent study, Chatchatee. et al. observed the development of tolerance of children aged <13 months receiving AAF including synbiotics or AAF alone [39]. Overall, 49% and 62% of subjects were CM tolerant by 12 and 24 months, of treatment respectively. However, no significant difference was found between the two groups. The rates of tolerance were found to be in line with the reported tolerance development trajectory [39]. Though the outcomes of these studies may be conflicting, the role of the gut microbiome in the development of tolerance to CMPA should not be underestimated.
It is clear that determining the resolution of CMPA through dietary avoidance is difficult because of the heterogeneous study populations and different methodologies and the several factors affecting the resolution of CMPA, including age, severity of initial reactions, and presence of multiple food allergies or other comorbid atopic conditions [39]. It is still important to discuss natural CMPA resolution in order to compare the effectiveness of new CMPA management strategies. The outcomes of the above studies exploring milk avoidance interventions are summarized in Table 2.  Infants aged <13 months with confirmed IgE-mediated CMA Subjects were randomized to receive AAF including synbiotics (AAF-S) (n = 80) or AAF (n = 89) for 12 months.
OFC to CM at 12 and 24 months.

The Milk Ladder and Baked Milk
The baking process alters the structure of different milk allergens, changing their stability and subsequently creating decreased allergenicity (through decreased IgE binding) [12]. In many cases, this is because of the destruction of conformational epitopes (antigenic determinants that bind with the IgE receptor) of milk proteins. Children with transient milk allergy are considered likely candidates to tolerate baked milk products [40]. Several studies have reported a good tolerance response to baked milk (BM) introduction in children, even reporting to accelerate tolerance to fresh milk [41,42]. This is supported by the observation that an increase in the intensity of IgG4 binding to CM epitopes occurred concurrently with a decrease in IgE-binding intensity among patients who recovered early from CMPA [43]. The production of IgG4 induces tolerance by blocking the binding of specific IgE to allergen [43]. Therefore, it can be hypothesized that the gradual introduction of denatured epitopes of baked milk proteins promotes the production of IgG4, thus inducing tolerance in milk-allergic patients ( Figure 2). OFC to CM at 12 and 24 months.

The Milk Ladder and Baked Milk
The baking process alters the structure of different milk allergens, changing their stability and subsequently creating decreased allergenicity (through decreased IgE binding) [12]. In many cases, this is because of the destruction of conformational epitopes (antigenic determinants that bind with the IgE receptor) of milk proteins. Children with transient milk allergy are considered likely candidates to tolerate baked milk products [40]. Several studies have reported a good tolerance response to baked milk (BM) introduction in children, even reporting to accelerate tolerance to fresh milk [41,42]. This is supported by the observation that an increase in the intensity of IgG4 binding to CM epitopes occurred concurrently with a decrease in IgE-binding intensity among patients who recovered early from CMPA [43]. The production of IgG4 induces tolerance by blocking the binding of specific IgE to allergen [43]. Therefore, it can be hypothesized that the gradual introduction of denatured epitopes of baked milk proteins promotes the production of IgG4, thus inducing tolerance in milk-allergic patients ( Figure 2).

Figure 2.
The mechanism of the baked milk ladder. Foods on the milk ladder undergo varying degrees of heat during the baking process. The major milk allergens, casein, betalactoglobuline, and alpha-lactalbumin, are denatured to varying degrees, with casein being the most heat resistant. This baking process alters the stability of these allergens and renders them less allergenic through decreased IgE binding. Regular consumption of these baked milk products causes the increased production of IgG4, which blocks the IgE binding to the allergen. Regular consumption of these baked milk products therefore decreases IgE levels and increases IgG4 levels, allowing for increased tolerance of milk proteins as the degree of allergenicity increases through the milk ladder.

Exploring the Introduction to Milk Using Baked Milk
One of the first studies in the literature to explore baked milk tolerance as a predictor of milk tolerance was by Nowak-Wegrzyn et al. in 2008 [41]. This study challenged the current standard of strict milk avoidance as the primary treatment for milk allergy, as the authors showed that patients who were tolerant to baked milk in an oral food challenge (OFC) showed significantly smaller immunologic reactions to fresh milk after 3 months of ingesting baked milk [41].
A further four studies explored the introduction of baked milk as a treatment for milk allergy from 2011-2018 [40,[42][43][44][45]. The amount of patients who became tolerant to unheated milk at the end of the studies ranged from 54% to 88.1% mean (62.5%) [42,45]. This compares to the range of those who underwent strict avoidance for the duration of this trial, which ranged from 0% to 66.7% (mean 21.42%) of those who tolerated unheated milk [42,45]. For these figures, the populations that were chosen to introduce baked milk to their diet had passed a baked milk OFC, whereas the groups who were instructed to strictly avoid milk were those who had failed a baked milk OFC. A strength of the study by Nowak-Wegrzyn A. et al. (2018) was that the authors gave a further unheated milk OFC to those who had passed the baked milk OFC, as those who passed this OFC were instructed to introduce all forms of milk and dairy into their diet, and those who did not were instructed to introduce a baked milk diet. It was not established by the other studies whether those who tolerated baked milk may also tolerate unheated milk during an OFC.
The three studies assessing home introduction to baked milk concluded that treatment of milk allergy using the introduction of baked milk was overall a safe method in patients [40,44,45]. It was found by Dunlop et al. that 35% of patients experienced an adverse reaction during treatment, the majority of which (77%) were characterized as mild [45], and Nowak-Wegrzyn et al. (2018) stated that no adverse reactions to baked milk at home were treated with epinephrine [44].
The literature on the home introduction of baked milk as a treatment for CMPA is limited, but it establishes that this approach is safe and effective in patients who tolerate baked milk. Each study gave detailed instructions to participants to begin home introduction with a baked milk product, a muffin in most cases, in increasing doses, and once tolerated, less baked goods should continue to be introduced in a sequential manner, from a pancake to baked cheese and, finally, to fresh pasteurized milk. However, instruction on the introduction of baked milk varies greatly, with up dosing periods ranging from every 2 months [45] to every 12 months [44]. These studies paved the way for the creation of patient-orientated guidelines for the stepwise progression from extensively heated to less heated milk in the home, known as the milk ladder.

The Creation of the Milk Ladder
The first published milk ladder was the Milk Allergy Primary (MAP) guideline in 2013 [46], indicated for mild to moderate non-IgE-mediated cow's milk allergy. This is a 12-step approach focusing on common British foods. An international (iMAP) version was published in 2017 in response to the increased uptake of the MAP Milk Ladder internationally [47]. This updated version takes healthy eating, feeding practices across the world, and other food allergies into account [47]. In doing so, they reduced the number of steps and removed high-sugar foods as necessary steps [47].
These ladders were designed with the aim of managing non-IgE-mediated food allergy, and therefore, a complete strategy and guideline using a milk ladder for IgE-mediated CMPA has not yet been established. However, its empirical use in several countries has increased in the last decade. A 2017 publication cited that 60% of physicians previously surveyed acknowledged using the MAP and the IMAP ladder for IgE-mediated allergies [48]. With the introduction of this ladder, other allergy societies, such as the Canada allergy society, have decided to adapt and create their own ladders [49]. This four-step ladder approach was adapted to include foods that were commonly consumed in Canadian house- holds and is intended for use by preschool children with a history of mild IgE-mediated reactions to milk.

Investigating the Effectiveness of the Milk Ladder for IgE-Mediated CMPA
The first study published regarding the use of the milk ladder in IgE-mediated disease was conducted by Ball et al. in the United Kingdom [50]. This was a retrospective study on the use of an adapted milk ladder management for IgE-mediated CMPA. The strategy uses four stages that start from baked milk products (biscuit) with increases to higher amounts with a progression of volume and levels of baked milk through all stages [50]. By stage four, the products are uncooked, and the ladder is finished with the introduction of typical pasteurized milk. Of the 86 patients retrospectively recruited, only 8 patients did not achieve tolerance of all dairy products.
Forty-three percent of patients presented with a mild to moderate allergic reaction during the milk ladder management, with no anaphylaxis diagnosis that required intramuscular adrenaline [50]. These reactions did not inhibit progress through the ladder, as parents reduced the baked milk dose as instructed and continued the progression again. The authors highlighted good compliance by the patients, with very few episodes of accidental or inappropriate diet exposure [50].
Their approach during this milk strategy included the use of not only baked milk products, but the necessity of low doses of each product at the beginning of each stage. This was a process of approximately 5 weeks, with the first introduction to baked milk being that of malted biscuits. The first phase starts with the ingestion of only a small crumb per day for a week (about 0.35 mg of milk protein), and then increasing the amount until a total of between 23 to 43 mg of milk protein is consumed [50]. The authors strongly encouraged starting reintroduction with very low doses and increasing slowly and with caution at each stage, and only progressing to a lesser baked stage once a substantial amount of the product at each stage could be tolerated [50]. They also promote a flexible approach to home-based introduction, eliminating the need for fixed recipes and measured doses that can be tiresome and may hinder progression and allow for varying factors such as the age of the child, lifestyle choices, parental choices, and food availability while maintaining safety [50].

A Prospective Analysis of the Milk Ladder and Strategies of Improvement
The latest study to evaluate the use of the milk ladder as a method of home introduction to milk for those with IgE-mediated CMPA was conducted by d'Art et al. and published in April 2022 [51]. Though this was a randomized trial to evaluate the progression through the milk ladder of infants who received a single dose of the elicited dose of milk (ED05) compared to the control group who did not receive a single dose of ED05, this is also the first published prospective study on the effectiveness of the MAP milk ladder for infants with IgE-mediated CMPA. Unlike previously described trials, this study excluded children who were already tolerating baked milk.
As a group overall, 37/57 (65%) were on step 6 at 6 months, and 13/57 (23%) were on step 12 at 6 months. This improved to 47/57 (82%) on step 6 and 31/57 (54%) at step 12 at 12 months. However, 24/37 (65%) of the intervention group had completed the ladder (step 12) compared to just 7/20 (35%) of the control group by 12 months, showing a significant difference in the rate of progression (chi sq 4.7, p = 0.03). The possible reason for this is the administration of the ED05, providing the parents with the confidence to initiate and progress through the ladder. They concluded that the very act of giving infants a single low dose of cow's milk in the presence of their mothers promoted parental confidence in home introduction, leading to accelerated progression up the milk ladder. This is supported by previous single-dose studies of ED05 of peanut and milk; it was evident that recruited families gained significant support and increased confidence from their participation [52]. This assumption was further supported in the study when it was shown that for the intervention group overall, there was a significant difference in maternal state anxiety between scores at baseline (M = 37.5, SD = 12.9) and at 6 months (M = 31.5, SD = 8.6); (t(32) = −2.81, p = 0.008), whereas no significant difference was found in the control group overall for maternal state anxiety for scores at baseline (M = 33.1, SD = 8.5) and at 6 months (M = 31.7, SD = 11.6); (t(14)4.17, p = 0.59) [51].
Though it was to be expected that some children would have mild symptoms when transitioning to a higher step on the milk ladder, no serious or unexpected adverse reactions occurred in children progressing through the ladder [51]. Three accidental exposures to milk occurred over the course of the study, all of these happening outside the home in childcare settings and relatives' houses [51].
Importantly, this is the first study to assess changes in maternal anxiety and food allergy quality of life (FAQOL) and its effect on progression through the milk ladder. This study shows for the first time that maternal trait anxiety was inversely associated with milk ladder progress in both groups, with poorer outcomes in children whose mothers had higher trait anxiety levels. Food Allergy Quality of Life scores improved in all groups by 12 months [51].
It can be concluded from the abovementioned studies that home introduction to milk using the baked milk ladder is an effective method of inducing tolerance to cow's milk proteins in the paediatric population, which thus improves food allergy quality of life [40,41,44,45,50,51]. However, with much of the treatment taking place in the home, the need for parent compliance is paramount to the success of the ladder [50]. For successful progression through the ladder to be achieved, a flexible ladder with multiple food choices at each step is needed, with occasional follow-up consultations occurring at key stages in the ladder to allow all families to incorporate the milk ladder into their daily lives [50]. With maternal anxiety correlating with progression through the milk ladder, methods to reduce anxiety, such as an initial dose of fresh milk and starting each step with the smallest amount of milk product, e.g., crumbs, should be encouraged [50,51]. The rates of the successful introduction of milk achieved in this study using the milk ladder protocol and in the previous studies discussed are displayed in Table 3 and graphically displayed in Figure 3.

Oral Immunotherapy
Standard protocols of OIT consist of different phases. After diagnosis through a positive OFC, the maximum dose of milk that can be consumed is found. Following this, a maintenance phase with daily consumption of the maximum tolerated dosage is conducted for 6-9 months. At the end of the maintenance dose, an OFC is performed testing desensitization. After a variable period of allergen withdrawal, a second OFC is performed testing sustained unresponsiveness. However, recently, it has been proposed that a lower-dosage endpoint is still enough to lower the risk of accidental ingestion with   Group 2: Routine care. Both groups implemented graded exposure to CM (using the 12-step milk ladder) at home.
Milk ladder position (out of 12) at 6 months and 12 months post randomization.
Step 12 at 12 months: Baked milk-reactive subjects avoided all forms of milk and were offered a repeat challenge ≥ 6 months from the initial challenge.
Baked milk-tolerant subjects incorporated baked milk products daily into their diets and after ≥6 months were offered challenges to baked cheese products.
Baked milk-reactive subjects were offered a repeat baked milk challenge > 6 months from initial challenge.
Baked milk-tolerant subjects were offered a challenge to baked cheese products > 6 months from initial challenge.
Similarly, after ≥6 months, baked cheese-tolerant children were offered challenges to unheated milk.
Subjects who incorporated dietary baked milk were 16 times more likely than the comparison group to become unheated milk tolerant (p < 0.001). Baked milk-tolerant, unheated milk-reactive subjects ingested heated milk products for 3 months and were then re-evaluated.
Unheated milk-tolerant subjects were instructed to add milk into the diet.
Baked milk-reactive subjects were instructed strictly to avoid all forms of milk.
Follow-up SPT and serum-specific IgE and IgG4 to milk, casein, and b-lactoglobulin measured at baseline and at 3 months in baked milk-tolerant, unheated milk-reactive group. Challenges were repeated at 6-or 12-month intervals over 36 months.
Overall, 41 (48%) children who ingested a baked milk diet became tolerant to non-baked milk; no difference was seen between 6 vs. 12 months' escalations.

Oral Immunotherapy
Standard protocols of OIT consist of different phases. After diagnosis through a positive OFC, the maximum dose of milk that can be consumed is found. Following this, a maintenance phase with daily consumption of the maximum tolerated dosage is conducted for 6-9 months. At the end of the maintenance dose, an OFC is performed testing desensitization. After a variable period of allergen withdrawal, a second OFC is performed testing sustained unresponsiveness. However, recently, it has been proposed that a lower-dosage endpoint is still enough to lower the risk of accidental ingestion with higher compliance and lower adverse events [53].
It is understood that OIT involves a reduction in mast cell and basophil mediator activation. However, the precise immunological mechanism is still to be established. The current evidence available is as follows. Repeated and frequent stimulation of basophils and mast cells results in a desensitization with increased tolerance of milk dosage. This stimulation produces a FoxP3+ Tregs production of cytokines such as interleukin (IL)-10, transforming growth factor (TGF)-β, and interferon (INF) È [54][55][56]. As a consequence, specific IgE levels decrease, and IgG 4 subclass (anti-inflammatory antibody) levels increase. Initially, B cells produce IgGM+, which then switches to IgG 3 . Next, there is a subclass switch from IgG 3 to IgG 1 to IgG 2 and then finally to IgG 4 due to alterations in the arrangement of the immunoglobulin-heavy chain locus.
It is believed that repeated allergen exposure, like that which happens in OIT, causes an IgG subclass switch from µ → È3 → È1 → ε producing IgE to µ → È3 → È1 → È2 → È4 producing IgG 4 [56]. IgG 4 acts to block antibodies by competing for allergen binding and inhibits the activation of basophils and mast cells. Moreover, the binding of the IgE allergen complex to CD23 is reduced by antigen-presenting cells and acts through FcÈIIb to inhibit IgE levels [57]. Finally, the production of IL-10, IFNÈ, and IL-6 from dendritic cells further depresses the allergic response, leading to tolerance [58]. As a result, T regulatory cell pathways are activated, and T helper 2 cell response is inhibited [59]. The proposed pathway of the achievement of tolerance to cow's milk proteins is demonstrated in Figure 4.

The Role of OIT and Its Implementation
Oral immunotherapy is defined as administering increasing doses of a food allerge (usually in a food vehicle) to an allergic patient in order to increase the threshold at whic they react to it [60]. It aims to reduce the risks following accidental ingestion and induc long-term tolerance or sustained unresponsiveness to the allergen. The first reported us of OIT on animals was conducted in 1909 [61]. The general structure of OIT protocols con sists of three phases: (i) day escalation, (ii) build up, and (iii) maintenance ( Figure 5). Du ing day escalation, six to eight doses of the allergen are administered at short intervals t the patient within the same day, in order to reach a "desensitized state". This is done un der observation in a clinical setting. In the buildup phase, there is daily home administra tion of the allergen with supervised scheduled dose increases every 1-2 weeks. Once target dose is achieved, home maintenance commences, and the target dose is consume daily. The maintenance phase ranges from months to years. An oral food challenge is con ducted post protocol to assess for the development of desensitization or sustained unre sponsiveness [62].

The Role of OIT and Its Implementation
Oral immunotherapy is defined as administering increasing doses of a food allergen (usually in a food vehicle) to an allergic patient in order to increase the threshold at which they react to it [60]. It aims to reduce the risks following accidental ingestion and induce long-term tolerance or sustained unresponsiveness to the allergen. The first reported use of OIT on animals was conducted in 1909 [61]. The general structure of OIT protocols consists of three phases: (i) day escalation, (ii) build up, and (iii) maintenance ( Figure 5). During day escalation, six to eight doses of the allergen are administered at short intervals to the patient within the same day, in order to reach a "desensitized state". This is done under observation in a clinical setting. In the buildup phase, there is daily home administration of the allergen with supervised scheduled dose increases every 1-2 weeks. Once a target dose is achieved, home maintenance commences, and the target dose is consumed daily. The maintenance phase ranges from months to years. An oral food challenge is conducted post protocol to assess for the development of desensitization or sustained unresponsiveness [62].
Currently, the EAACI advises starting OIT for children aged 4-5 years with persistent cow's milk allergy [63]. Although several clinical trials on cow's milk OIT have been conducted, a standardized protocol is yet to be established by the EAACI. The trials so far have differed in the type of product, dosage, and duration. All the studies displayed in Table 4  Currently, the EAACI advises starting OIT for children aged 4-5 years with persistent cow's milk allergy [63]. Although several clinical trials on cow's milk OIT have been conducted, a standardized protocol is yet to be established by the EAACI. The trials so far have differed in the type of product, dosage, and duration. All the studies displayed in Table 4

Results of OIT Trials for CMPA Regarding CM Intake
Three studies investigating desensitization/tolerance after OIT all maintained their target OIT dose as 200 mL cow's milk (CM) [66,73]. Meglio [75]. Efron et al. reported that 70% of the OIT group was able to tolerate all types of milk and dairy products apart from raw milk. In contrast to the others, Efron et al. saw allergy resolution in both the control and treatment groups. However, they found decreased resolution time and age in patients in the treatment group as compared to the control group [78]. In Kauppila et al.'s subjects, 56% could maintain daily milk intake post protocol [67]. Gruzelle et

Results of OIT Trials for CMPA Regarding CM Intake
Three studies investigating desensitization/tolerance after OIT all maintained their target OIT dose as 200 mL cow's milk (CM) [ [75]. Efron et al. reported that 70% of the OIT group was able to tolerate all types of milk and dairy products apart from raw milk. In contrast to the others, Efron et al. saw allergy resolution in both the control and treatment groups. However, they found decreased resolution time and age in patients in the treatment group as compared to the control group [78]. In Kauppila et al.'s subjects, 56% could maintain daily milk intake post protocol [67]. Gruzelle [65]. High OIT completion rates were reported by both the oral challenge test group and the fixed starting dose group in a study by Calvo et al. [64]. Ebhirami et al. reported desensitization in 92.9% [80]. Overall, relatively high success rates were reported with OIT, but it is important to note that their target/end OIT doses varied. A summary of the proportion of subjects who achieved desensitization/tolerance to cow's milk following the OIT protocols in these studies is displayed in Figure 6.      [71]. Similarly, in Gruzelle et al.'s study, patients reacted at higher doses on their last OFC than their first OFC [79]. challenge test group and the fixed starting dose group in a study by Calvo et al. [64]. Ebhirami et al. reported desensitization in 92.9% [80]. Overall, relatively high success rates were reported with OIT, but it is important to note that their target/end OIT doses varied.. A summary of the proportion of subjects who achieved desensitization/tolerance to cow's milk following the OIT protocols in these studies is displayed in Figure 6.  [71]. Similarly, in Gruzelle et al.'s study, patients reacted at higher doses on their last OFC than their first OFC [79].
Apart from desensitization/tolerance achievement, a common and salient result across protocols was adverse reactions. The number of reactions occurring differed in each study. For instance, Gruzelle et al. noted 33.3% of their patients had reactions, and 53% of patients in Goldberg et al.'s protocol were unable to continue OIT due to IgE reactions [77,79]. Mota et al. reported 45% had reactions during their maintenance phase [75]. Skripak et al. noted a significantly higher frequency of reactions among their OIT group (35% per patient) versus their placebo group (1% per patient) (p value = 0.02). However, 90% of these reactions required no treatment [69]. In Longo et al.'s study, epinephrine administration was required four times, with two cases needing emergency care [70]. Ebrahimi et al., too, found that two patients needed IM epinephrine administration during OIT treatment, and that rhinoconjunctivitis was the most common reaction following the build-up phase [80]. Calvo et al. concluded in their study that OIT is a safe treatment for CMPA patients, and it should be offered to families immediately after diagnosis [64]. Apart from desensitization/tolerance achievement, a common and salient result across protocols was adverse reactions. The number of reactions occurring differed in each study. For instance, Gruzelle et al. noted 33.3% of their patients had reactions, and 53% of patients in Goldberg et al.'s protocol were unable to continue OIT due to IgE reactions [77,79]. Mota et al. reported 45% had reactions during their maintenance phase [75]. Skripak et al. noted a significantly higher frequency of reactions among their OIT group (35% per patient) versus their placebo group (1% per patient) (p value = 0.02). However, 90% of these reactions required no treatment [69]. In Longo et al.'s study, epinephrine administration was required four times, with two cases needing emergency care [70]. Ebrahimi et al., too, found that two patients needed IM epinephrine administration during OIT treatment, and that rhinoconjunctivitis was the most common reaction following the build-up phase [80]. Calvo et al. concluded in their study that OIT is a safe treatment for CMPA patients, and it should be offered to families immediately after diagnosis [64].
Moreover, some studies found patients who underwent OIT developed long-term tolerance to CM. Longo et al. and Martorell et al., who conducted follow ups 1 year post protocol, found that 11/30 and 23/27, respectively, from their treatment groups remained tolerant to CM and could continue on unrestricted diets [66,70]. Mota et al. reported 92% of patients following unrestricted diets post protocol [75]. Maeda et al. also observed that at the 2-year follow up, seven out of eight patients experienced no reactions when consuming CM doses greater than 100 mL [71,76].

Factors Potentially Influencing OIT Results
Some studies have suggested external factors that potentially affected the results of the OIT protocols. Amat et al. found that patients tolerating a higher CM dose at baseline achieved higher tolerance threshold doses at follow up [76]. These patients achieved a 1802 mg tolerance threshold, whereas more sensitive patients only achieved a tolerance of 104 mg. Mota et al. additionally found a correlation between a patient's baseline history of anaphylaxis and the allergic reactions they faced in the maintenance phase (p value < 0.001) [75]. Relationships between patients' immunological profiles and outcome of OIT were also observed. Takahashi et al. reported significant differences in initial serum CM-IgE, casein-sIgE, and β-lactoglobulin-sIgE levels and asthma severity between those in the treatment group who achieved sustained unresponsiveness and those that did not [74]. Kauppila et al. found that a patient's baseline IgE level affected OIT protocol continuance [72]. Five studies additionally administered antihistamines, which may have impacted the number of adverse reactions occurring during the protocol [70][71][72][73]. However, Meglio et al. showed that eight patients achieving target doses remained asymptomatic even 2 months after stopping cetirizine [68]. Interestingly, Maeda et al. conducted transcriptome analysis after specific antigen stimulation on two patients who were non-responders and two patients who were responders to their OIT protocol. Certain milk-specific genes that were up/down regulated were identified in each group. This study has suggested that the identified genes could be chosen as antigen-specific markers to predict OIT outcomes in advance. This is an area requiring further research, as the sample size used by Maeda et al. is too small to be conclusive [71]. Table 4 summarises the outcomes of studies exploring oral immunotherapy interventions for CMPA.

Early Studies in Early Introduction
As described, the use of CMPA OIT is commonly started in patients between 4-6 years, with little research done in patients less than one year old. The first introduction of this type of management was in 2011 by Reche et al., who performed a case-control study of 20 patients comparing strict avoidance and an OIT protocol in children with a mean age of three, with a one-year follow up [81]. After this period, all OIT patients were tolerant to milk in comparison to three that were tolerant in the control group (p = 0.003) [81]. Despite the small sample size, this study created a foundation for larger and more rigorous study of the use of early introduction to milk in young infants in later years.
In the years following, three more studies have investigated the use of early introduction OIT in young infants (Table 5). Lee et al. compared children with challenge-proven CMPA between 7 and 12 months of age who underwent oral desensitization to milk and those who underwent strict avoidance of milk for 6 months [82]. This study again suggests that introducing milk at an early age may induce tolerance in children who may potentially not acquire tolerance naturally as they grow older [82].

Establishing Early Introduction in Young Infants as a Management Strategy for CMPA
The previously mentioned study by Berti et al. explored the efficacy of an early introduction protocol designed to be conducted at home for children aged >12 months. This involved children taking, every day for three to four weeks, the higher dose of milk already tolerated in hospital during an OFC [65]. If parents reported a steady tolerance of the dose of milk offered at home, a doubling dose of milk was administered under medical supervision. If the doubling dose was tolerated in the hospital, parents were instructed to continue offering the same doubled dose at home for another three to four weeks until the next hospital evaluation. Every increase in the dose of milk was initially tested in hospital, in order to favor child safety, until a tolerance of 40 mL of milk was stably achieved by infants. Once the infant had reached tolerance to 40 mL of CM without reactions at home for at least two weeks, families were instructed to increase the dose by 5 mL every week, up to 50 mL tolerated, then to increase the dose by 10 mL every week, up to 100 mL, and then 10 mL every 3 days, up to 150 mL of milk [65].
A total of 66 (97%) of children reached the target of 150 mL milk, with a median time to achieve the protocol of 5.5 months (IQR: 4.5-7, range: 3.5-16) [65]. The study found a high rate of compliance among families and suggests that a home-based introductory protocol for young infants may prove to be a safe and effective solution for families compared to the usual method of strict avoidance [65]. The largest study to evaluate the effectiveness of early introduction using an oral immunotherapy protocol in young infants was conducted by the previously mentioned Calvo et al. [64]. In this retrospective analysis conducted between 2007 and 2018, 335 infants under 1 year of age with IgE-mediated CMPA were treated with early introduction to milk at the moment of diagnosis [64]. Two methods of early introduction were explored. Between January 2007 and June 2011, an in-hospital oral challenge test (OCT) was performed, with gradual increases in infant formula doses, seeking the threshold dose for the start of treatment (OCT group). From July 2011 to June 2018, OIT was introduced with a fixed starting dose (FSD = 0.5 mL), without prior challenging (FSD group) [64]. The starting dose was administered at home at least twice a day for a week. Patients who did not suffer any adverse reactions made dose increments every half week, whereas patients who suffered frequent adverse reactions were proposed to make a 2-week dose increase, with a tolerated dose of 150-200 mL infant formula considered to be a successful treatment [64].
A total of 67 patients in the OCT group (98.5%) and 262 (98.1%) in the FSD group completed the treatment successfully and continued to consume dairy products regularly. The median duration of immunotherapy was 106 days (range 27-269) in the OCT group and 77 days (range 77-254) in the FSD group, showing a significant difference between groups (p value 0.001) [64].
Reche et al. found only one infant needed to be pre-treated with antihistamines, and four out ten patients had very mild side effects over the course of the 12-month OIT protocol [81]. During the study by Calvo et al., 45.4% presented some type of adverse reaction, none of which were considered severe, with most of these reactions happening in the hospital during the up-dosing visit [64]. Furthermore, no patient was required to attend the emergency room after home treatment [64].
These findings show that even in the earliest days of infancy, early introduction using oral immunotherapy is an effective management strategy for CMPA and can be safely performed in the home [81].

Early Introduction or the Natural Development of Tolerance?
Finally, the main point of discussion regarding early introduction to milk in young infants is whether the patients were going to achieve a spontaneous resolution of CMPA disease with or without this treatment. The two studies that featured control groups of infants undergoing strict avoidance to milk found a significant number of children achieved tolerance after undergoing early introduction compared to the control at the end of the trial [81,82]. However, it was not established whether the children in the control group would later develop a natural tolerance to cow's milk. The three studies that have considered this subject concurred that it is too early to know, and that further long-term data and new research are needed to establish the effectiveness of early introduction to milk compared to the development of natural tolerance [64,65,82]. Table 5 summarises the outcomes of studies exploring methods of early introduction of milk using oral immunotherapy protocols for CMPA.

Discussion
The scientific literature on CMPA has continuously increased since the 1930s [12]. This has led to new treatment strategies that have been a feature of clinical practice in recent decades [12]. A summary of the milestones in the development of treatment for CMPA is shown in Table 6. Table 6. Milestones of the development of treatment of CMPA.

1909
First recorded use of OIT in animals. [61] 2001 Allergenic IgE and IgG antibodies for b-and k-casein and alpha(s1)-casein epitopes were identified, with higher levels of these IgE antibodies associated with persistent CMPA. [17,78,83,84] 2002 A significant number of infants who consume AAF achieve tolerance by 20.5 months. [37] 2004 Children aged 6 years or more with severe CMPA found to be tolerating a daily intake of cow's milk following 6-month OIT protocol. [68] 2008 Significantly smaller SPT mean wheal diameters and significantly greater casein-IgG4 concentrations were shown in CMPA patients who ingested baked milk products for 3 months. [41] 2011 Subjects who incorporated dietary baked milk were more likely to become unheated milk tolerant.
[40] Infants with mean age of 3 months with CMPA who underwent an OIT protocol became tolerant to milk.
[47] Consumption of Lactobacillus rhamnosus probiotic with eHCF promotes tolerance to cow's milk. [38] 2018 Clinical trial evidence showing improved tolerance to fresh milk following baked milk introduction compared to strict avoidance. [42,44,45] 2019 First study to assess the effectiveness of the milk ladder in IgE-mediated CMPA, with most children completing the ladder and tolerating almost all dairy products. [50] 2020 Large trial in infants under 1 year showing increased tolerance to cow's milk following OIT protocol. [64] 2022 Parental anxiety correlates with progression through the milk ladder. [2] To objectively compare the effectiveness of each of the three strategies is not possible at this stage due to the lack of clinical trials comparing these treatments in the management of CMPA. However, the effectiveness of the treatment in resolving CMPA depends on multiple factors.
Dietary avoidance is easy in theory but challenging to be applied. It involves extensive involvement of the dietician to provide guidance on foods to avoid that contain traces of cow's milk. Even if labeling of food with proper indications is mandatory by law in most countries, including the EU, accidental exposure still occurs. Contamination of food in restaurants, canteens, and other settings is possible. One study followed 80 patients with CMPA until the achievement of tolerance or up to the age of 18 years, finding accidental ingestion of milk in at least a third of them [85]. In addition to the constant risk of anaphylaxis, the avoidance of milk may have significant nutritional implications [4]. Sinai et al. compared adult height in 87 patients with CMPA compared with 36 individuals with no dietary limitations and found that patients with lifelong CMPA had an average 3.8 cm lower adult height than controls [86]. Patients with CMA also have higher rates of vitamin D deficiency [87].
The milk ladder has been shown to be a safe and effective method of introducing baked milk and thus promoting the acquisition of milk tolerance [50]. Though earlier studies suggested that efficacy depends on severity of CMPA [45], with only baked milk-tolerant patients considered for gradual reintroduction, the most recent studies have shown that the milk ladder is safe in children with an allergy to uncooked pasteurized milk [2,50]. Though it can be suggested that the milk ladder improves quality of life as a home-based treatment using readily available milk products, progression through the milk ladder requires a high level of parent compliance and sufficient education and reassurance regarding mild allergic symptoms that may appear as the child progresses through the ladder [50]. Parental anxiety is a significant factor in progression, and a single-dose challenge is a cost-effective and timeefficient manner of reducing this anxiety [2]. Though the safety of the milk ladder as a form of introduction to cow's milk protein has been shown by several studies, further prospective cohort studies are necessary to determine whether this method merely introduces milk as the child is acquiring natural tolerance, or whether progressing through the milk ladder actively immunomodulates the CMPA in these children who would otherwise continue to be sensitized to cow's milk.
Oral immunotherapy provides a comprehensive and systematic method of attaining tolerance through four key stages: escalation, up dosing, maintenance, and withdrawal, with high rates of success reported in most studies. A starting dose of milk is established through an oral food challenge, and exact dose regimens are provided for each stage, which may prove easier to follow than the milk ladder for some families, where the quantity of milk proteins given at any time may vary due to differences in cooking preparation and serving provided. However, parents and patients must commit to at least three oral food challenges over the course of the treatment, which may increase the burden of treatment on families compared to the milk ladder, which can be entirely provided for and monitored at home. Early introduction is a novel way of introducing milk to young infants using an OIT protocol [64,65,81]. However, the practicality of this strategy may be a barrier to implementing it in some clinics, where waiting lists to attend allergy specialists may be up to a year long, and waiting periods for oral food challenges even longer in many cases, and many children would not have the opportunity to undergo OIT in infancy.
A significant factor when considering treatments for CMPA is safety and the risk of severe allergic reactions. Though mild allergic symptoms are frequent findings in the milk ladder and OIT protocols, the incidence of severe allergic reactions from accidental exposure to milk is far less than those who are strictly avoiding milk. Parents should, however, receive education from allergy specialists regarding mild allergic symptoms as their child moves to higher doses and should be encouraged to continue progressing through the milk ladder or OIT protocol, and they should be aware of the signs and symptoms of severe allergic reactions [50]. A number of OIT studies reported that a small number of patients required IM adrenaline [70,80], whereas IM adrenaline was not required by any patient due to foods consumed on the milk ladder [2,50]. However, further prospective studies must be carried out in order to investigate whether there is a significant difference in the safety of these two treatment strategies.

Conclusions
This review of the literature discussed the mechanism, efficacy, and safety of three important management strategies of IgE-mediated CMPA in children: complete avoidance, the milk ladder, and oral immunotherapy. Early introduction using an oral immunotherapy protocol was also discussed as an emerging management strategy. Though the milk ladder and early introduction to milk using oral immunotherapy may provide methods of introduction to milk for CMPA, further studies should prospectively compare the effectiveness and safety of these treatments for IgE-mediated CMPA compared to the natural acquisition of tolerance to milk.