Since birth we are exposed to dangerous microbes than put our life at risk. So, we need several cells and molecules capable of defending our body. We will call "immune system" to our body defenses, and "immunocytes" to the immune cells that protect us. Some people have a vital organ or tissue severely damaged. They need a transplant to continue living. The immunocytes are very strong and powerful to attack bad microbes. However, they must tolerate foreign molecules in the transplanted graft. In this book we will learn about the importance of transplantation to prolong life in individuals with a badly damaged organ or tissue. Transplantation saves lives