How long does the effect of birch pollen injection SIT on apple allergy last?

Background: Recent studies showed that injection specific immunotherapy (SIT) with birch pollen extract greatly reduces or cures the associated apple allergy in a large proportion of birch pollen-allergic patients. However, the long-term efficacy of SIT for apple allergy has not been assessed. Objective: To evaluate the duration of the effect of injection SIT with birch pollen extract on apple allergy in birch pollen-allergic patients. Methods: Thirty birch pollen-allergic patients showing both the clinical disappearance of apple allergy and a negative SPT with fresh apple at the end of their injection SIT course were followed-up at 12-month intervals from 6 months after SIT was stopped. Apple tolerance as well as SPT was assessed on all occasions. Fifty-seven birch pollen-allergic subjects without apple allergy and not submitted to SIT regularly followed-up for the onset of oral allergy syndrome (OAS) were used as controls. Results: The overall prevalence of OAS after 30 months of follow-up did not differ between patients and controls. Although most patients became re-sensitized to apple by SPT over time, >50% of them were still able to tolerate eating the fruit at the 30-month follow-up visit. Conclusion: Although most patients show a 'natural', gradual propensity to apple re-sensitization (a consequence of prolonged and repeated inhalation of birch pollen responsible for primary sensitization?), the clinical effects of injection SIT on food allergy seem rather long lasting.