Patients who undergo kidney transplant must take medications to prevent organ rejection. There are standard immunosuppressant medications such as prednisone, tacrolimus (Prograf), mycophenolate mofetil(Cellcept) or sirolimus (Rapamune) that are given to patients to prevent rejection. It is well known that patients on immunosuppressant medications are at increased risk from viral infections, such as influenza. However, it is not well understood how immunosuppressive medications may uniquely affect the immune response to infection. This study will determine whether there are unique differences in the effects on the immune system by these different immunosuppressive medications, particularly differences between tacrolimus and sirolimus.