Innate and adaptive immunity play important roles in immunosurveillance and tumor destruction. However, increasing evidence suggests that tumor-infiltrating immune cells may have a dual function: inhibiting or promoting tumor growth and progression. Although regulatory T (Treg) cells induce immune tolerance by suppressing host immune responses against self- or non self-antigens, thus playing critical roles in preventing autoimmune diseases, they might inhibit antitumor immunity and promote tumor growth. Recent studies demonstrate that elevated proportions of Treg cells are present in...