Finding the safe place between the hammer and the anvil: sounding the depth of therapeutic immunosuppression.

Transplanted organs express allogeneic epitopes against which a recipient's immune system mounts a destructive response. In the absence of a reliable strategy for induction of donor-specific tolerance, therapeutic immunosuppression remains the best prospect to prevent rejection, but exposes recipients to increased risks of infection and cancer. Defining the individual upper and lower limits between which the transplanted patient is safe requires the validation of biomarkers able to depth-sound the recipient's immune system.