A pancreas transplant is an organ transplant that involves implanting a healthy pancreas (one that can produce insulin) into a person who has diabetes. Because the pancreas performs functions necessary in the digestion process, the recipient's native pancreas is left in place, and the donated pancreas attached in a different location. In the event of rejection of the new pancreas, the recipient could not survive without the native pancreas still in place. The healthy pancreas comes from a donor who has just died or it may be a partial pancreas from a living donor. Whole pancreas transplants from living donors are not possible, again because the pancreas is a necessary organ for digestion. At present, pancreas transplants are usually performed in persons with insulin-dependent diabetes who have severe complications.

From http://en.wikipedia.org/wiki/Pancreas_transplantation