Diabetes can alter the body’s immune system and reaction to both allergy symptoms and allergy medications.

Allergies don’t affect blood sugar directly, but some medications may.

References

Diabetes and Itching – Causes and Treatment Options
By Ginger Vieira On May 9, 2020
https://diabetesstrong.com/diabetes-itching/

Treatments to relieve allergy symptoms may impact blood glucose.
Pamela Daniels, Michigan State University Extension – May 15, 2020. https://www.canr.msu.edu/news/treatments_to_relieve_allergy_symptoms_may_impact_blood_glucose_1

Allergy Tips for People With Diabetes. 
By Christina Boufis. Medically Reviewed by Brunilda Nazario, MD on January 15, 2014. https://www.webmd.com/diabetes/features/diabetes-spring-allergies