The purpose of this study is to review and learn the connection between the immunospressive drugs treatment and its effects on the success of the solid organ transplantation and the dangers it withholds (opportunistic infections), plus understanding the importance of an accurate dose of those drugs given to the patients and finding the right balance in order to have a successful transplantation as well as a healthy patient. Furthermore, learning about the importance of searching for novel drugs, capable of increasing the tolerance (increasing graft survivability), while keeping the immune system of the patient functioning (protect patient from opportunistic infections).