Background
IgE-mediated food allergy affects up to 8% of infants and children in industrialized countries and often starts early in infancy (1). Especially infants with eczema are at high risk for developing food allergies and it is the current understanding that sensitization occurs via the cutaneous route due to an impaired skin barrier function (2, 3). Accordingly, a high peanut consumption in the household has been shown to be a possible risk factor for developing peanut allergy in infancy (4). Moreover, we demonstrated that food proteins, such as peanut and hen’s egg can be found not only in the eating area, but also in bed dust and increase with consumption, further supporting the hypothesis of cutaneous sensitization (5, 6). Therefore, German S3-guidelines on allergy prevention recommend that peanut allergy should first be ruled out in infants with moderate to severe atopic dermatitis, before introducing peanut into the infant´s diet for preventive purposes (7). What about other food allergens such as tree nuts? It has been shown that infants with eczema are at risk for multiple food sensitizations (8). During the last decades, nutrition across Western countries has changed substantially as for example vegan and plant-based diets have become a growing trend (9). Tree nuts, such as cashews, hazelnuts and walnuts are a nutritional mainstay of plant-based diets (10). Moreover, plant-based alternatives for milk and milk-products often contain tree nuts and such products are gaining in popularity among consumers (11). These changes in dietary habits may lead to a wider spread of tree nut allergens in households, increasing the risk for cutaneous exposure in infants. Moreover, tree nuts are very potent allergens, as evidenced by having caused up to 21% of all fatal anaphylaxis cases among children between 1992 and 2018 in the United Kingdom (12). Among tree nuts, hazelnut, cashew and walnut are the most common triggers inducing anaphylaxis in children in Europe (13). Regarding the clinical reactivity, sensitization to 2S albumins of peanut and tree nuts have been shown to be associated with severe allergic reactions (14). Ana o 3, the 2S albumin of cashew, discriminates between allergic and tolerant children better than cashew-specific IgE and probability curves for Ana o 3-specific IgE have been calculated, a 95% probability could be estimated at 2.0 kU/l (15). Similarly, the 2S albumin of hazelnut, Cor a 14, estimates the probability for a positive clinical reaction, with a 90% probability for hazelnut allergy at 47.8 kU/l (16). Concerning walnut, the 2S albumin Jug r 1 has been shown most accurate for estimating the risk for clinical relevant walnut allergy (17). There are hints, that individuals with peanut allergy have a higher likelihood of being allergic to tree nuts compared to the general population (18, 19). Therefore, the question comes up what to recommend in a peanut-allergic child: To eat, to screen, or to avoid as it has been recently discussed in this journal (20)? To date, there are scarce data on the sensitization patterns to tree nuts in very young infants and children with peanut sensitization. Therefore, the aim of this study was to investigate, how often peanut-sensitized infants and toddlers are sensitized to cashew, hazelnut and walnut and their seed storage proteins, which might be associated with a high risk for clinical reactivity. We hope to add some important information to this discussion.