In recent years, it has been highly recommended that unclear cases of food allergy should be confirmed by standardized food challenges, such as the DBPCFC, for definite diagnosis although it carries a risk of severe adverse events, such as anaphylaxis [19, 20]. In the present study, we demonstrate for the first time that the measurement of serum IgG4 antibodies as well as IgE antibodies to hen’s egg white is of value in the prediction of reactivity to hen’s egg during food challenges, and therefore helpful in the confirmation of food tolerance in egg-sensitized children. The titers of food-specific IgE antibodies have already been reported to be useful in the prediction and possible diagnosis of IgE-mediated food allergy; several studies have proposed “cut-off” values of antigen-specific serum IgE or weal diameter sizes for the diagnosis of food allergy, to optimize the use of food challenge tests [21, 22]. Sampson and Ho identified PPV and NPV values of specific IgE antibodies against different foods, such as milk, egg, and peanut, using the CAP-FEIA system . They found that, in the population mainly referred for severe atopic dermatitis, egg-specific IgE cut-off levels for >95% PPV, >90% PPV, and >90% NPV were 6 kUA/L, 2 kUA/L, and 0.6 kUA/L, respectively. This study has been followed by others, although the PPV, NPV, and cut-off values varied considerably [23–25]. Compared to these studies, the PPV and NPV of IgE to egg white in our study were relatively low. Both the sensitivity and AUC for egg white-specific IgE in our study were also low compared to 2 earlier studies [23, 24]. These differences may have arisen because the patients with atopic dermatitis were excluded in our study group or because the enrolled studied population was relatively older than that of previous studies. Whatever the reason, we feel that a more sensitive and more specific marker is clearly needed for the prediction of reactivity to hen’s egg during food challenges.
Recent studies on immunotherapy with inhalant allergens suggest a protective role of IgG4 antibodies during treatment [26, 27]. It has been suggested that allergen-specific IgG4 antibodies act as blocking antibodies by competing with IgE for allergen binding to IgE receptor-expressing cells, such as mast cells and basophils , and competition between IgE and IgG4 antibodies at the level of antigen-presenting cells has also been detected in vitro. The production of both IgE and IgG4 antibodies is up-regulated by IL-4 produced from activated Th2 cells . However, IL-10 secreted by regulatory T cells during immunotherapy potentially suppresses IgE production and simultaneously increases IgG4 production . Another immune response with a protective effect against the development of allergic disease is a modified Th2 immune response that includes high levels of IgG4 antibodies in the absence of IgE antibodies .
Several reports have described the role of specific IgG4 in natural tolerance development in food allergy. Stapel et al. have shown that food-specific IgG4 does not indicate food allergy or intolerance, but rather indicates immunological tolerance, linked to the activity of regulatory T cells . Ruiter et al.  have reported similar results in cow’s milk allergy showing that IgG4 levels were the highest in atopic subjects who were tolerant to cow’s milk, whereas Shek et al.  have described an increased IgG4 response in children allergic to cow’s milk compared to atopic patients without suspected food allergy. Ahrens et al.  reported no difference in hen’s egg white-specific IgG4 levels between tolerant and allergic children. The discrepancy between our study and Ahrens’s result may be due to the following reasons: (1) the age of the study population, (2) the challenged materials, and (3) the procedure of the challenge test, (4) the exclusion of atopic dermatitis patients. The main reason for the discrepancy may be the age of the population. The mean age in our study was 5 years, although it shows 2 years of age of its study. We also observed a lower level of AUC for hen’s egg white-specific IgG4 in the under 5 years of age group than in the over 5 years of age group. Children younger than 2 years of age have less ability to produce IgG4. Thus, it remains unclear as to whether the role of IgG4 in food allergy is related to tolerance or repeated exposure to food ingestion. In our study, children demonstrating a lower ratio of IgE/IgG4 to hen’s egg (i.e., relatively higher IgG4 to IgE) were more likely to be able to eat eggs. The cut-off level of IgE/IgG4 for more than 90% of the NPVs was 1, and that for 20% of the post-test probability values was 13.5, based on the calculation of negative likelihood ratios (0.2). This means that 90% and 80% of patients who showed cut-off levels of IgE/IgG4 that were less than 1 and 13.5, respectively, were able to eat hen’s eggs.