I had Remicade about 20 years ago with UC. It put me into remission. Fast forward to jpouch and fistulas. Last year I was given Remicade again after having been tested for antibodies (negative) but developed serum sickness a couple of weeks after the first infusion. The symptoms all cleared and I didn't continue on Remicade.
The fistula was likely caused by an undetected leak at the anastomosis but in the unlikely event that the fistulae were as a result of Crohns I was given a trial of Humira. I had a much worse reaction to it ("lupus like syndrome" and some likely permanent nerve damage in my legs).
I know I sound like such a Debbie Downer but even though most people have great success, some of us just don't respond well with biologics.