I think allergies and immunotherapy, while sometimes better than nothing, leave a lot for pharmaceutical R&D! It sounds like it works fantastically for some, and range from modest efficacy to none for others (count me in the “none” category).
Specific to your question, you can desensitize to something, then re-sensitize to it later. The body makes no promises to desensitize forever! Obviously, in theory, you desensitize to it then you’re desensitized forever, but in practice, that is not always the case