Atopy’s immunologic properties change with age in asthma

By Peter Sergo, medwireNews Reporter

The strength of interaction between atopy and elevated eosinophil levels in asthma decreases with age, a study finds.

"The interaction… was not homogeneous across age categories," write Samuel Arbes (Rho Federal Systems Divisions Inc, Chapel Hill, North Carolina, USA) and colleagues. "Analyses of increasing age categories showed a transition from essentially complete interaction between the factors to complete independence."

The degree of interaction between atopy and elevated eosinophil levels was strong in cases of children's asthma, but subsequently became absent in adults above the age of 55 years.

"[This] suggests different mechanistic pathways for these factors by age and supports the notion that asthma is a heterogeneous disease," the authors observe in Clinical and Experimental Allergy.

The research team assessed data on 7207 patients who were 6 years of age or older and reported having current asthma at the time of the National Health and Nutrition Examination Survey from 2005 to 2006.

Patients with atopy were defined as those who developed immunoglobulin E (IgE) antibody blood concentrations of at least 0.35 kU/L in response to exposure to at least one of 19 allergens that included aeroallergens (n=15) and foods (n=4).

The researchers categorized the percentage of white blood cells that were eosinophils as either being "low" (below the median of 2.4%) or "high" (above the median).

Overall, statistically significant synergistic interactivity on an additive scale between atopy and elevated eosinophil level in current asthma was present among patients aged 6-17 and 18-40 years with an excess prevalence of 15.3% and 10.8%, respectively. The strength of this relationship decreased, such that it was no longer found in adults who were at least 55 years of age (-0.2%).

The proportion of the youngest age group whose asthma was attributable to the interaction of these two factors was 94%, whereas no cases in the adults above the age of 54 years indicated such an interaction.

"In children, atopy and a high eosinophil level act jointly through at least one common causal pathway, whereas in the oldest adults, the 2 risk factors act independently through separate pathways," the authors explain.

"The examination of independence and departures from independence on an additive scale can provide insight into causal mechanism or pathways," Arbes and team add.

The apparent age-dependency of how these immunologic properties interact in atopic asthma carries implications for the treatment of asthmatic patients, with either IgE or a high eosinophil level being targeted, they conclude.

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