Allergen immunotherapy requires long-term small-dose exposures which cause an immunoglobulin class switching resulting, in effect, a "masking" of the allergen with the non-allergy-inducing globulin. It requires small doses over an extended period of time: no bolus infusion (e.g., "I'm having my big cup of pollen tea now") is going to do it, and it's not going manifest over a short period of time ("and 20 minutes later I feel better.")

Additionally, a general fault of immunotherapy for allergy is that we tend to present the allergen in a manner distinct from its biological presentation - e.g., oral presentation of inhaled allergens. Since we have different mucosa-associated lymphoid tissues throughout the body, and since they do react differently, generally you're not going to get a great response to oral administration of a non-oral allergen (e.g., something you'd normally inhale, like pollen).

GP said "starts acting in 10 minutes" Immunotherapy doesn't work that way.

This might be it. Very interesting!!