Here you can see how complicated the T cell interaction in acute rejection and where the glucocorticoids, antimetabolites and calcineurin inhibitors work to block this reaction and prevent the graft from being rejected.
The cell that contains the foreign material (top) presents this material to a waiting T lymphocyte (bottom) and the receptors of each cell interacts. This sets up a signal between the cells that makes the responding cell activated. This activated T cell is what the immunosuppressants, including the antimetabolite (Mycophenolate Mofetil), calcinuerin inhibitor (Cyclosporine or Tacrolimus) and steroid must stop. They can work at many of the signal pathways that are working within the responding T cell.