Passenger leukocytes and microchimerism: what role in tolerance induction?

The role of passenger leukocytes in determining the outcome after transplantation is complex. In some settings, donor-derived passenger leukocytes can initiate graft rejection, whereas in others they contribute to graft acceptance. Both donor and recipient factors contribute to this potential dual role. Understanding the interaction between passenger leukocytes and the recipient's immune system, particularly after liver transplantation, may provide important clues for developing novel strategies for inducing specific unresponsiveness to donor alloantigens.