Monoclonal antibodies (MAb) have been shown to be effective in inducing immune tolerance in transplantation and autoimmunity. Several different MAb have tolerogenic properties and their effect has been studied in a range of experimental animal models and, in some cases, in clinical trials. The tolerant state seems to be maintained by CD4⁺ regulatory T cells (Treg), induced in the periphery, capable of suppressing other T cells specific for the same antigens or antigens presented by the same antigen presenting cells. Furthermore, following the initial induction of Treg cells under MAb treatment, Treg cells themselves can maintain the tolerant state in a dominant way in the absence of the therapeutic MAb or other immunosuppressive agents, and are able to recruit other T cells into the regulatory pool—a process named infectious tolerance.