We do have people on the forum who have experienced this type of allergic reaction to Humira, but their reaction developed relatively slowly (over hours), giving them time to get to hospital.
It is possible, likely even, that you will have your first injections under medical supervision, as you (I presume) need to be taught how to give yourself injections, so don't despair or worry too much: you may not be alone if something goes wrong. If you really are worried, I suggest asking a friend, relative or neighbour to sit with you while you inject, or actually asking if you can have the first injections under medical supervision, just to put your mind at ease.
Your doctors should know about
your fistula already, surely?
Co-Moderator Crohn's Forum. Medications for Crohn's
~~ Diet and Nutritional Therapy for Crohn's