Background: A recent study carried out in North of Milan, an area that was recently colonized both by birch and ragweed pollen, showed that subjects developing allergies to these “new” allergens were much older than those becoming allergic to “traditional” airborne allergens, which suggests that predisposition to develop respiratory allergies is probably allergen-specific, persists throughout life, and does not occur until the patient encounters the appropriate allergen for a sufficiently long period.

Objective: The present study aimed to test this hypothesis by following-up a large cohort of patients monosensitized to airborne allergens.

Methods: The prevalence and characteristics of new sensitizations to airborne allergens after >2 years of followup were retrospectively assessed in 726 patients monosensitized to grass, pellitory, mite, birch, ragweed, Alternaria, mugwort, or plantain living North of Milan.

Results: The overall prevalence of new sensitizations was 35% (256/726), with marked differences between the subgroups ranging from 11% in Alternaria-allergic subjects to 46% in grass-allergic subjects. Patients allergic to grass, birch, or pellitory pollen showed a significantly higher propensity to become sensitized to a second airborne
allergen than subjects allergic to ragweed, Alternaria, and mite. Overall, the “new” allergens (birch and ragweed) caused 228/256 (89%) new sensitizations detected in the whole study group. Ragweed pollen induced 90% (38/42) of new sensitizations in birch pollen allergic patients, and birch pollen induced 80% (8/10) of new sensitizations
in ragweed allergic patients. No difference in age at the first visit or in duration of the follow-up between patients developing and not developing new respiratory allergies was observed.

Conclusion: This study shows that: 1) predisposition to develop respiratory allergies is allergen-specific and persists throughout life; 2) proneness to become allergic to certain airborne allergens might be associated with primary sensitization to specific airborne allergens; 3) in monosensitized adults, sensitization to another airborne allergen that has been always present in that particular geographical area is unlikely.

Key Words: Respiratory allergy, Ragweed, Birch, Sensitization, Follow-up, Genetics, Predisposition, Atopy