T regulatory (Treg) cells are a subset of T lymphocytes which is characterized by CD4+ / CD25+ / FOXP3+. These naturally occurring Treg cells originate in the thymus, and comprise 2-10% of peripheral CD4+ T cells. It has been shown that Treg cells are able to inhibit T cells proliferation and cytokine production and play critical roles in preventing autoimmunity as well as in controlling tumor immunity and transplantation tolerance. Impaired Treg function or Treg cell deficiency will develop variety of autoimmune diseases, while higher frequency of Treg cells will cause hypo-immune response to pathogens.