In the last decade there has been an increasing interest for the action of regulatory T cells (Treg) in preventing transplant rejection, autoimmunity and other inflammatory diseases and in maintaining dominant tolerance. It is becoming clear that such regulatory function does not rely simply on direct inhibition of "aggressive" T cells. In fact, several studies suggest that Treg cells may induce changes in the target tissue, promoting a state of "immune privilege" where protective genes such as heme oxygenase-1 (HO-1) and indolamine 2,3 dioxygenase (IDO) may have a critical role.