IgE-sensitization at age 24 years
At age 24 years 43.4 % (970/2,234) were sensitized to any of the tested foods or airborne allergens. This proportion was in the same range as at the 16-year follow-up, and higher compared with ages four and eight years, Figure 2. Sensitization to foods had decreased compared to previous follow-ups affecting 8.4 %, while sensitization to airborne allergens was more common at the 24-year follow-up, affecting 42.2 %. Thirty-five percent were sensitized to airborne allergens only, 1.3 % to foods only and 7.1 % both to foods and airborne allergens. Timothy and birch were the most prevalent sensitizing allergens (26.6 % and 24.2 %, respectively), Figure 3. The median level of sIgE for foods was 2.5 kUA/l (25th percentile 1.2 kUA/l, 75th percentile 13.5 kUA/l) and for airborne allergens 6.4 kUA/l (25th percentile 2.4 kUA/l, 75th percentile 14.7 kUA/l).
Longitudinal analyses - impact of sex on IgE-sensitization over time, up to age 24 years
We evaluated the association between sex and IgE-sensitization over time using generalized estimating equations (GEE) with females as the reference category and results are shown in Figure 4. Sex was not significantly associated with sensitization to foods up to 16 years. However, at 24 years significantly more males than females had IgE-sensitization to foods. Evaluation of the whole time period revealed no statistically significant association between sex and IgE-sensitization to foods (over all OR: 1.10, 95% CI 0.93 - 1.32), Figure 4. In contrast, male sex was significantly associated with IgE-sensitization to airborne allergens at all ages (over all OR: 1.68, 95% CI 1.46 - 1.94).
IgE-sensitization to specific allergens at 4, 8, 16 and 24 years in relation to sex
The prevalences of IgE-sensitization to specific food allergens among females and males at ages 4, 8, 16 and 24 years are shown in Figure 5. Significantly more males were sensitized to milk and wheat at age 24 years. However, there were no significant differences between females and males regarding IgE-sensitizations to specific foods at ages 4 years, 8 years and 16 years using a stringent Bonferroni correction cut-off (p=<0.004). With respect to airborne allergens, more males than females were sensitized and for many airborne allergens there were significant differences (p=<0.004) between females and males already at age 4 years. At age 24 years significantly more males than females were sensitized to timothy, birch, cat, dog, mugwort, mites and horse (all p<0.004). Comparison of sIgE-levels for the 14 allergens between females and males at age 24 years showed no significant differences, (all p>0.004), Table S1. The highest level of sIgE was against birch for both females (median level 6.5 kUA/l (25th percentile 1.5 kUA/l, 75th percentile 28.0 kUA/l) and males (median level 13.0 kUA/l (25th percentile 2.3 kUA/l, 75th percentile 40.0 kUA/l), and tended to be higher among males, p=0.005. Sex was not significantly associated with sIgE-levels at ages 4, 8 and 16 years, all p>0.004, data not shown.
Sensitivity analysis
At the 24-year follow-up more females (n=1,244, 61%) than males (n=990, 48%) attended the clinical examination and provided blood for analysis of sIgE. In view of this sex-related difference in response rates we performed a sensitivity analysis and compared the prevalence of IgE-sensitization, atopic dermatitis, asthma and rhinitis5 at previous follow-ups for females and males who attended the clinical examination at age 24 years with the original cohort, Table S2. Overall, differences were minor for both females and males, although atopic dermatitis, rhinitis and IgE-sensitization were somewhat more common, especially at recent follow-ups, among both females and males who attended the clinical examination at the 24-year follow-up. However, differences were not more pronounced among males.
We also explored potential differences between males and females regarding some environmental exposures early in life. In the study population, 83% attended kindergarten at the 2-year follow up. Differences between boys and girls were only minor (p=0.19). Further, we found that a similar proportion of girls and boys had a cat or dog at home both at inclusion (2 months), at age 1 year, 2 years and 4 years (all p-values non-significant).