Rapamune, also known by its generic name sirolimus, is a medication primarily used in the prevention of organ rejection in patients who have undergone kidney transplantation. It functions as an immunosuppressant, working by suppressing the body's immune response to prevent it from attacking the transplanted organ. Sirolimus does this by inhibiting the activation of T cells, a type of white blood cell that plays a central role in immunity. This medication is part of a class of drugs known as mTOR inhibitors, which block the action of the mammalian target of rapamycin, a protein that regulates cell growth, proliferation, and survival. While effective in preventing organ rejection, Rapamune requires careful monitoring due to potential side effects and interactions with other medications. It is typically used in combination with other immunosuppressive drugs to optimize transplant outcomes.