One of the advanced forms of surgery that a patient may undergo is an organ transplant. These are necessary when organ failure occurs to ensure survival, and are made possible by all the gracious organ donors who have chosen to help those in need.
On the other hand, organ transplants don't always go as planned. Sometimes the body will reject the organ it's given because it's "different." While organ rejection is common, researchers haven't always fully understood why it occurs.
Perhaps the most important thing to know is that the immune system is designed to attack foreign entities that the body isn't accustomed to, and when you introduce a new organ made up of someone else's cells, the body's immune system goes on the fritz.
New and innovative ways to block this false alarm from occurring are under development so that organ rejections will become less common. On the other hand, research is still in progress, as the discovery of what tips off the immune system and leads to organ rejection is still quite new.