Schematic of extracellular and intracellular T cell–signaling pathways targeted by immunosuppressive medications. The IL-2 receptor (IL-2R) and CD28 molecules are shown; daclizumab and CTLA4-Ig, respectively, block interaction of these cell-surface receptors with their ligands. OKT3 interacts with the TCR-associated CD3 complex. Rapamycin inhibits cell cycle progression through its interaction with mTOR. Cyclosporin A and tacrolimus inhibit calcineurin, thereby inhibiting NFAT and IL-2 synthesis. Azathioprine, through its 6-ThioGTP metabolite, inhibits CD28 signals.