My 9 year old girl (very active and healthy) breaks out with the occassional large white, raised welt on her arms or legs or back. It is a raised and itchy area, much like a really bad mosquito bite...except there aren't any mosquitos around her when this happens. I am thinking some sort of allergic reaction and give her an antihistamine for relief. But is there any way to prevent this from occuring in the first place?