The existence in humans of circulating T regulatory (Treg) cells able to recognize exogenous antigens such as Der p1 allergen or bacterial streptokinase (SK) was investigated. CD4+CD25highFoxp3+ circulating T-cells from healthy nonatopics and cultured with Der p1 loaded dendritic cells (DCs) but not those with unloaded or SL-loaded DCs, suppressed the proliferative response to Der p1 of autologous Der p1-specific T-cells from the CD4+CD25-Foxp3-subset. Functionally active Der p1-specific Treg cells were obtained from a pool of circulating CD4+CD25high Foxp3+ T-cells of Der p1-sensitive atopics (Maggi, L. et al. J Allergy Clin Immunol 2007, 120(2): 429-436).