Background: Data on anaphylaxis due to allergen immunotherapy (AIT) are limited. This study assessed AIT-induced anaphylaxis using data from the European Anaphylaxis Registry. Objective: To analyze the characteristics, symptoms, severity, and emergency management of AIT-induced anaphylactic reactions across age groups and the administered allergenic source. Methods: Data were obtained from 54 allergy centers across 10 European countries from 2007 to 2023. Anaphylactic reactions involving at least 2 organ systems were analyzed for symptoms, severity, associated diseases, administration routes, and emergency treatment. Statistical methods included χ² and Fisher's exact tests. Results: AIT accounted for 1.1% of the cases in the anaphylaxis registry (173/15,748), of which 1.8% were pediatric and 0.7% were adults. Respiratory symptoms were more frequent in children/adolescents (92%) than in adults (66%) among AIT-related anaphylaxis, whereas cardiovascular and gastrointestinal symptoms were more common in adults (40% vs 78% and 20% vs 42%, respectively). Only a few sublingual immunotherapy-related cases were documented, including 2 grade III reactions, with no fatalities (sublingual immunotherapy [n = 8] vs subcutaneous immunotherapy (SCIT) [n = 153]). One fatality due to SCIT was reported (grass pollen). Delayed reactions (>30 minutes) were reported in 22 cases, predominantly after SCIT. All delayed grade III reactions that occurred more than 120 minutes were reported in children. Adrenaline was underused in emergency management and administered in 30% of grade II and 50% of grade III reactions. Conclusion: Our study highlights the importance of age-specific monitoring and appropriate emergency treatment to enhance patient safety during AIT.