Patients after kidney transplantation often take sirolimus orally because of its ability to counteract organ rejection, and are often advised to take sirolimus along with cyclosporine or corticosteroids. The main reason then why sirolimus has anti-organ rejection effects is that sirolimus inhibits the activation and proliferation of antigens and cytokines, such as interleukin 2, interleukin 4 and interleukin 15 inspired T lymphocytes, through a mechanism of action that is very different from that of other immunosuppressive agents. Sirolimus inhibits antibody production in cells where sirolimus binds to a pro-immune protein, FK-binding protein 12, to generate an immunosuppressive complex. This sirolimus FK-binding protein 12 complex has no activation of calcineurin. This complex binds to and inhibits the activity of mammalian sirolimus target molecules and is able to block the proliferation of cytokine-driven T cells.