Student: read previous posts in the thread and you'll get all the explanation you want.
Autopilots generally control pitch using elevator trim, and can "disguise" a serious out-of-balance situation that's developing due to icing by just dialing the trim slowly out to the stops...
I don't fly the Q400 so I can't comment on how its' autopilot works but the medium and heavy jets I fly use the elevators to control pitch and then use the trim system to remove those inputs. I've yet to see any guidance from my company, my certifying authority or the manufacturer of my type to suggest that I should disconnect the autopilot in icing conditions - should I be concerned?