**3.1 Food allergies**

This hypersensitivity to particular proteins present in food, known as allergens, occurs when the immune system erroneously perceive foreign proteins as dangerous, initiating an allergic immune reaction [29].

The most common type of food allergy is mediated by immunoglobulin E (IgE), and is estimated to have an impact in the life of 5–8% of the children and up to 4% of the adults worldwide [39]. Food allergic reactions mediated by IgE comprise distinct phases; the allergic sensitization, where the food antigen is taken up, processed and displayed on the surface of antigen presenting cells (APCs); which, in the presence of interleukin-4 (IL-4) and/or IL-13, provide signals for the activation of the T helper 2 (TH2) subtype of T cells. Then, TH2 cells in conjugation with IL-4 and IL-13, will induce class switching in B cells, which differentiate into plasmocytes (antibody-producing cells) that secret allergen-specific IgE [40]. After the allergic sensitization, the subsequent re-exposure (elicitation phase) to the allergen, will now result in a more robust immune response. Here, the antigen-specific IgE binds to the surface receptor FcεRI expressed on mast cells and basophils. The cross-link of the FcεRI receptors with IgE will trigger mast cells and basophils degranulation, which leads to the secretion of inflammatory mediators e.g. β-hexosaminidase and histamine. In addition, allergen-induced cytokines (IL-4 and IL-13) are also released fostering the typical food allergic symptoms, which can range from mild to a life-threatening allergic reaction (anaphylaxis) [39, 40]. Given that, the incidence as well as the severity of food allergy, is gradually increasing, the search for novel therapeutics to mitigate this condition is in high demand [29].

As described earlier, there are various immune mechanisms implicated in food allergy that may, therefore, be targeted in prospective anti-allergic strategies. In this light, the extraordinary structural characteristics, wide distribution in fruits and vegetables, and the well-studied anti-inflammatory and anti-oxidant properties of PC, make these bioactive compounds fitting candidates for anti-allergic therapies [41]. In fact, various studies with PCs have suggested that some of these metabolites, especially phenolic acids and flavonoids, may exhibit certain anti-allergic benefits and although the exact mechanisms behind their action are not clear, data shows that PC can intervene at both the allergic sensitization and the elicitation phases [42, 43]. Moreover, PC can also modulate gut microbiota and potentially influence food allergy [44].

Several methodologies are currently in use to evaluate the capacity of distinct PC to interact with specific allergen proteins. Plundrich et al. performed in silico analyses to narrow down the search for PC present in cranberries/ and or lowbush blueberries (rich in anthocyanins), which could theoretically interact with Ara h 2, the most pro-allergenic protein in peanuts, specifically in the region that is thought to be the binding site for IgE [45]. This screening, in concert with further *in vitro* experiments revealed that procyanidin C1 and chlorogenic acid could potentially interact with Ara h 2 inducing conformational changes, which masked the IgE epitope [45]. Covalent interactions between chlorogenic acid and ovalbumin (OVA), the major allergen found in the egg white, also induced modifications in OVA conformation, resulting in the direct shielding of the linear IgE epitope, which attenuated allergic mechanisms [46]. Accordingly, histamine release experiments, showed that the basophil degranulation was inferior in human basophils sensitized with the OVA conjugated with chlorogenic acid when compared to OVA unconjugated, implying a decrease in the crosslinking of the FcεRI receptors via IgEallergen interaction [46]. Also, the ability of phenolic compounds to bind to dietary allergen is pointed out as having a beneficial effect due to precipitation events [47]. Accordingly, Yichen Li et al. observed that PC from pomegranate juice could form stable complexes with cashew nuts, thus reducing allergen recognition by antibodies, and consequently the immunoreactivity to cashew nuts [47].

Despite not completely mimicking the human pathophysiology of food allergy, animal models of food allergy are important pre-clinical research tools for the food industry. These models are elected due to their capacity to simulate the most common reactions observed after the exposure to specific allergens, namely IgE production, TH2 related cytokine expression and mast cells degranulation [48, 49]. In fact, various animal models are now used to study the effect of PC as modulators of allergy. For example, the metabolites derived from epicatechin found in the circulating plasma of a mouse model of OVA allergy fed with phenolic compounds extracts isolated from apple/ or purified epicatechin, were associated with the reduction of several clinical allergic symptoms [49]. Additionally, in the ileum, the mRNA levels of the TH2 – related cytokine IL-13 and the pro-inflammatory cytokine IL-12 were decreased as well, suggesting that epicatechin could be a possible modulator of allergic reactions [49]. Also, Abril-Gil et al., used Brown Norway rats to investigate the potential protective effect of cocoa diets, which contain high amounts of flavanols (e.g. epicatechin, catechin and procyanidins) in allergic immune reactions upon OVA re-exposure [50]. Strikingly, their findings showed that while in the group of rats deprived of cocoa high levels of serum specific anti-OVA IgE were observed, in the other groups where cocoa was offered, IgE was significantly lower [50]. Moreover, the *in vitro* analysis of spleen and mesenteric lymph node cells (MLN) cytokines secretion revealed that IL-5 and IL-13 were reduced in the MLN, and that in the spleen, IL-4 was also reduced in a specific cocoa diet. Interestingly, the cocoa diet was also important for attenuating degranulation events by reducing the FcεRI and mast cell mediators (proteases) gene expression and release [50].

More recently, a mouse model (C3H/HeJ mice) representative of peanut allergy was used to evaluate the capacity of PC rich-extracts obtained from blueberry and cranberry to minimize the allergenicity of peanut proteins [51]. Here, colloidal aggregates composed of PC extracts (with different percentages) and proteins derived from peanut were introduced in the diet of peanut sensitized mice for several weeks, before challenging the mice with a higher dose of peanut flour. At the end of the experiment, it was observed that mice pre-treated with the PC aggregates, showed reduced IgE and IgG levels; and lower expression of the allergeninduced basophil activation protein marker CD63 in spleen lysates, when compared to mice kept on a diet with non-complexed peanut proteins [51].

The manifestly promising results demonstrated by these and other *in vivo* experiments, suggest that in the presence of PCs, the re-exposure to allergens result in less exuberant allergic responses. In this way, the use of these bioactive compounds hold promise to surmount the immune responses triggered by the oral administration of food antigens, contributing therefore, to the oral tolerance to dietary proteins [52].

In summary, the urgent need for effective therapeutics for food allergy and given the complex mechanisms involved in this escalating pathology, diverse antiallergic strategies are being explored. In this perspective, the natural ability of PC to interact with food allergens, interfere with IgE interaction or production, reduce the secretion of allergic mediators and modulate the expression of allergy related cytokines make PCs attractive agents for mitigating food allergy.
