Injection immunotherapy with different airborne allergens did not prevent de novo sensitization to ragweed and birch pollen north of Milan

Background: Previous studies suggest that allergen-specific immunotherapy (SIT) with a single allergen may prevent sensitization to other airborne allergens.

Objective: To assess the effectiveness of injection SIT in preventing sensitization to birch and ragweed pollen north of Milan, a geographical area that was recently colonized by these two airborne allergens.

Methods: 691 adult patients monosensitized to grass (n = 342), pellitory (n = 76), ragweed (n = 66), birch (n = 112), and house dust mite (n = 95) were studied. 284 (41%) of them received injection SIT as part of routine outpatient care; the remaining 407 (59%) not submitted to SIT, served as controls. All patients underwent follow-up visits after no less than 2 years in order to detect possible new sensitization to birch and/or ragweed.

Results: 227 (33%) patients became sensitized to birch and/or ragweed pollen during the follow-up period. Surprisingly, the prevalence of new sensitizations was significantly higher among subjects receiving SIT (132/284; 46%) than among those not receiving SIT (95/407; 23%; p < 0.001). 27 subjects developed new sensitization to birch and/or ragweed pollen while undergoing SIT. These findings were consistent in all subgroups with different airborne allergies.

Conclusion: In this study, injection SIT did not exert any preventive effect against denovosensitization to the two novel airborne allergens, birch and ragweed pollen, in adult monosensitized patients.