In the present study the investigators intend to investigate if and how the consumption of birch pollen-related food allergens affects birch pollen allergy. In other words, the investigators are interested to analyse whether Bet v 1-related food allergens activate Bet v 1-specific memory cells and thus, contribute to the maintenance of the pollen allergy outside the pollen season. Data obtained in this study will help to clarify the immunological and clinical role of cross-reactivity between pollen and food allergies and will reveal whether avoidance of such foods should be recommended for the patients. Finally, novel approaches for diagnosis and therapy of pollen-related food allergens can be developed.