Multiple Mast Cell Tumors/Allergy Testing

My 3 yo boxer has had two MCT removed in the last 3 months and another bump has appeared. All the reading I have done suggests that MCT can develop from elevated histamine levels. He does have seasonal allergies that we try to treat with Benadryl. We are scheduled for allergy testing next week. I am hoping that the vet can identify what he is allergic to and develop a vaccine to keep the histamine levels in check and reduce the recurrence of a MCT. Has anyone had any experience with multiple MCT and/or allergy testing? Thanks in advance!