The present invention relates to methods and kits for identifying, quantifying and isolating regulatory T cells, to methods and kits for diagnosing or monitoring autoimmune diseases, immunoinflammatory diseases, allergic diseases, predispositions thereto, infectious diseases, cancer, cancer treatment and/or organ transplantation based on regulatory T cell quantity, to methods and kits for predicting responses to therapy for autoimmune diseases, immunoinflammatory diseases, allergic diseases, predispositions thereto, infectious diseases, cancer and/or organ transplantation based on regulatory T cell quantity, and to methods and kits for therapy using isolated regulatory T cells.