This Swedish longitudinal population-based study shows that almost one in two young adults (43.4%) is sensitized to at least one specific allergen. Sensitization to airborne allergens continues to increase with age up to young adulthood, whereas sensitization to food allergens seems to level off. Sensitization to airborne allergens was significantly more prevalent among males than females from early childhood up to adulthood, while there were only minor differences for food allergens.
The prevalence rates of IgE-sensitization over time in our study are similar to European studies with a follow–up time of 18 and 19 years7,8. However, our study goes beyond childhood and adolescence and we show that the overall prevalence of IgE-sensitization remained rather unchanged from late adolescence up to age 24 years. A new finding of our study is that even though male sex was strongly associated with IgE-sensitization to airborne allergens at all ages, there was little evidence for significant associations between sex and sensitization to foods. Most large population-based longitudinal studies included IgE-sensitization to airborne allergens7–9, 14,31 while few included both foods and airborne allergens8,9,16. Moreover, when evaluation of differences in IgE-sensitization between females and males was done, sensitization to foods and airborne allergens were often analyzed together. Salo et al explored IgE-sensitization to foods in relation to sex among 1–5 years old children in a large population-based cross-sectional study from the US4 and found no significant difference, which is in line with our results. However, in the same study male sex in individuals 6 years or older was significantly associated with IgE-sensitization to one or more foods, in particular shrimp and peanut4. The authors also noted that IgE-sensitization to house dust mites and birch was significantly more common among males 6 years or older. Part of the association between IgE-sensitization to shrimp and peanut among males is probably explained by serological IgE cross-reactivity with house dust mites and birch, respectively32,33. Similarly, some of the sex differences regarding wheat sensitization at age 24 years found in our study could likely be explained by serological IgE cross-reactivity with timothy34. Thus, we believe that the statistically significant difference in IgE-sensitization to wheat between females and males at age 24 years is a reflection the high prevalence of IgE-sensitization to timothy among males at 24 years. The observed sex-differences in IgE sensitization to milk at 24 years is difficult to interpret due to low numbers of affected individuals. The IgE assays used where almost identical over time. Thus, they are unlikely to have impacted differences between follow ups or differences in IgE sensitization between females and males.
The discrepancy regarding the associations between sex and IgE-sensitization to foods and airborne allergens we found is intriguing and can probably not be explained by general differences in allergen exposure, although we know that the intake of some dietary components, such as antioxidants, may differ between males and females35.
We also explored whether sIgE levels differed between males and females. We found no significant differences although sIgE to birch was higher in males (median 13.0 kUA/L vs 6.5 kUA/L, borderline significant after multiple test correction). Our findings are similar to results from Salo et al (N = 7,268) who compared sIgE levels for 19 sIgEs in relation to sex among participants aged 6 years or older. Only two sIgEs (milk and Aspergillus fumigatus) differed by sex (both p < 0.05 and > 0.001)4.
Genetic factors are known to be strongly associated with the risk of developing sensitization in children36. Also, epigenetic changes are linked to atopy and high total IgE levels37. Sex-specific genetic effects on sensitization and allergic diseases have been reported in the literature38–40 but no consistent picture or explanation of the underlying biology has emerged. Whether there are primarily genetic, hormonal or environmental factors associated with the observed sex differences in IgE-sensitization in our study remain to be further investigated.
Strengths of our study include the population-based design, long follow-up time, limited loss to follow-up and that blood was collected for analyses of sIgE including both food and airborne allergens at four time points. We applied a stringent multiple-test correction approach in order to identify robust differences in IgE-sensitization between males and females. Similar to most longitudinal cohort studies, potential selection bias needs to be taken into account in our study, especially since more males than females were lost to follow up at the recent 24-year follow up. We therefore evaluated whether selection bias differed between females and males. The willingness to participate was somewhat higher among individuals with IgE-sensitization, atopic dermatitis and rhinitis especially at older ages. However, difference between females and males were minor. Thus, selection bias is unlikely to explain the sex differences found in our study.
In summary, we report that IgE-sensitization to airborne allergens continues to increase with age up to young adulthood, whereas sensitization to food allergens seems to level off. At all ages, sensitization to airborne allergens was more common in males compared to females. Further analyses of the underlying determinants for the differences in IgE sensitization between females and males are warranted.