Jane Brody in the NY Times wrote an excellent introduction to urticaria (hives): both the acute and chronic variants.
"Hives of one sort or another afflicts about one person in five at some time during their lives. For most, the problem is short-lived or “acute,” lasting less than six weeks. A specific cause, or trigger, can usually be identified. This is not the case for chronic urticaria."
Brody's article, "Infinite Itch" will answer many of your questions. It was published on May 8, 2012.