**4. Allergy**

Food allergy (FA) is an unexpected reaction resulting from an immunological alteration, in which a specific and reproducible immune response is triggered by the ingestion of food antigens normally tolerated by the population [62].

Although the prevalence rate of FA is variable in relation to age and geographical location, about 10% of the population is affected by FA, with a prevalence in childhood and in developing countries [63–65]. All foods can cause FA, but the most commonly involved are peanuts, cow's milk, hen's egg, tree nuts, fish, shellfish, wheat, seeds, and soy [66]. The essential process for avoiding the development of FA is oral tolerance. It derives from oral exposure to food antigens and is mediated by dendritic cells (DCs), able to stimulate the differentiation of naive T cells into positive forkhead box P3 (Foxp3) T cells that produce IL-10, leading to the inhibition of sensitization to specific food allergens. Proinflammatory cytokines produced by intestinal epithelial cells in association with pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs) lead to the production of inflammatory cytokines that switch antigen-presenting cells into a proinflammatory phenotype, increasing Th2 cells, that drive the allergic response through the production of IL-4, the expansion of eosinophils and mast cells and the isotypic switch of B cells towards the production of IgE. The onset of FA is linked to the breakdown of oral tolerance. The first phase of sensitization, at the first contact with the antigen, leads to the production of specific IgEs, which are anchored to specific receptors on mast cells and basophils. At the second contact with the allergen, the cells are activated by the binding of the antigen to the IgEs, releasing various mediators including histamine, TNFα, platelet activation factor (PAF), leukotrienes, IL-4, IL-5, IL-9, IL-13, IL-31, and IL-33, which lead to a range of symptoms from the skin to life-threatening one [67,68].

Increasing evidence suggests that the gu<sup>t</sup> microbiome contributes to the pathophysiology of such inflammatory disorders [69–72]. In particular, dysbiosis is associated with the pathogenesis of food allergies (Table 1) [73–84].


**Table 1.** Association between the most frequent food allergy and microbiota.

Bacteroidetes and Firmicutes makeup 90% of the microbiota and are involved in the pathogenesis of FA. The association with FA has been identified especially for *Clostridia* species, able to increase the production of Treg, with resulting inhibition of allergic inflammation and promotion of oral tolerance. *Bacteroides fragilis*, a species of Bacteroidetes, was also capable of producing polysaccharide A (PSA), which increased the suppressive capacity of Treg cells and the production of IL-10 by Foxp3 + T cells in a murine study. Several studies identify a lower abundance of bacterial class Clostridia, phylum Firmicutes, in children with food allergy compared to healthy children [3].

Fazlollahi M. et al. studied 141 children with egg allergy compared with healthy controls, highlighting a preponderance of Lachnospiraceae, Streptococcaceae, and Leuconostocaceae in the early gu<sup>t</sup> microbiome of children with egg allergy. The association found between the presence of the families of *Lachnospiraceae* and *Ruminococcaceae* and egg sensitization has, therefore, led the authors to identify the early diversity of the microbiota in egg-sensitized children as a target for preventive or therapeutic interventions [81].

Intestinal microorganisms such as *Clostridium leptum*, *Eubacterium rectal*, and *Faecalibacterium prausnitzii*, are able to produce SCFA, whose fermentation produces butyrate, propionate, acetate, and valerate with a higher concentration in the colon. SCFAs have direct immune-modulatory effects and are a key factor in promoting immunological tolerance towards harmless antigens and preventing inflammation (Figure 3).

**Figure 3.** SCFAs effects in immunological tolerance: induce gu<sup>t</sup> dendritic cells (DC) to express retinal aldehyde dehydrogenase (RALDH), with result in the production of retinoic acid that upregulates expression of the gut-homing integrins α4β7 on peripheral regulatory T cells (Treg); promote immune tolerance, regulate the antibody response through the production of IgA and IgM; stimulate the production of anti-inflammatory mediators such as IFNγ and IL-10, which induce the expansion of Treg cells and the suppression of proinflammatory T helper 17 (Th17) and Th2 cells; reduce the production of proinflammatory cytokines including IL-1β, IL-6, and IL-17.

In addition to the production of SCFA, intestinal bacteria also produce polyamines (PA) (spermidine, spermine, putrescine, cadaverine) essential for maintaining the intestinal barrier function through upregulation of junctional proteins.

The rise of food allergy in modern society has led to the postulation of the hygiene hypothesis, according to which lack of early childhood exposure to infectious agents suppresses the development of the immune system with the rise of atopic diseases. Recent work has revisited the hygiene hypothesis model to include mode of delivery, antibiotic intake, diet, and synthetic chemicals as factors in altering gu<sup>t</sup> microbiota [3]. After the introduction of the complementary diet, the composition of the microbiota varies according to the diet applied with evident compositional differences between a diet rich in fiber, characterized by *Alistipes*, *Bilophila*, and *Bacteroides*, and higher abundance of *Roseburia*, *Ruminococcus*, and *Eubacterium*, and diet rich in fat. In general, *Lactobacillus*, *Clostridium*, *Ruminococcus*, *Peptostreptococcus*, and *Bacteroides* are species that, through the catabolization of tryptophan, are able to regulate the immune response and the proliferation of T lymphocytes with consequent induction of the expression of the IL-10 receptor-1 (IL-10R1) and inhibition of proinflammatory cytokines [85]. In this context, it is clear that a diet that is higher in fat but low in fiber, like the Western one, maybe the cause of the increased prevalence of FA in Western countries [86].

About that, Mckenzie et al. have described the "nutrition-gut microbiome-physiology axis", an essential link between diet, gu<sup>t</sup> microbiota, and allergic diseases. It was also shown that food diversity was associated with greater expression of Foxp3, suggesting a protective effect of a diversified diet against the development of FA. On the contrary, reduced Foxp3 expression was present in children with a less diversified diet [1].

Data supporting the ability of the microbiota to influence allergic sensitization was found in mice treated with antibiotics and GF, which developed greater allergic sensitization than controls. In particular, Jiménez-Saiz R et al. demonstrated that eosinophil-deficient GF mice had intestinal fibrosis and were less prone to allergic sensitization than GF controls, establishing the role of the microbiota in regulating the frequency and activation of eosinophils in the intestinal mucosa [87].

More commonly, Clostridiales and Lactobacillales appear to have beneficial effects on food tolerance, while Bacteroidales and Enterobacteriales have ambivalent effects. In particular, children with AD and FA had a microbiota more colonized by *Escherichia coli* and *Bifidobacterium pseudocatenulatum* and less by *Bifidobacterium adolescentis*, *Akkermansia muciniphila*, *Bifidobacterium breve*, and *Faecalibacterium prausnitzii* compared to children with AD without FA. The authors, therefore, established an association, also in this case, between early colonization with potentially more pathogenic bacteria, such as *Clostridium difficile* or *Stafilococcus aureus*, and the development of FA, and *vice-versa* colonization with more beneficial bacteria such as *Bifidobacteria* and food tolerance [1]. In particular, studies in the literature have revealed the desensitizing effects of *Lactobacillus rhamnosus GG* in cow's milk and in peanut allergy [3,88,89].

The World Allergy Organization (WAO), regarding probiotic supplementation, concluded that there was weak evidence to support their action in reducing the risk of developing allergic disorders in pediatric patients but that nevertheless, a small reduction in risk could be connected [90].
