M.S.
I used to work in an allergy office and when we gave immunotherapy, the patient was required to stay 30 minutes afterwards, in case of reaction. If they didn't, they had to sign a waver (even then, it was people who had been doing this for years and years and were very aware of reactions and what to look for/do for it). Did you stay for 30 minutes? If it was the patient's first, we may have even had them stay longer. Can't remember, this was 14 years ago. Anyway, 30 minutes is generally the time frame in which a reaction will occur. That's not to say that you couldn't have a reaction later. Exercise shortly after an injection can bring it on, or a lot of stress. But, that is what the Epi-pen is for. Again, this was 14 years ago, but I don't remember ever hearing of anyone saying they had a reaction after they left the office. Not saying it didn't happen, but I don't remember any. We had an occasional reaction in the office after a shot, but we were ON it, and the patient was treated immediately. That's why you wait there.
I'm sure the nurse told you all this, but if you miss a scheduled injection, they are supposed to take you down a dose or not increase you the next time. So, being able to go to each one on time, is important. The more consistent you are, the better the treatment will work for you.
I've seen many people get great results from immunotherapy, so good luck!