Naturally arising CD4+CD25+ T regulatory cells actively maintain immunological tolerance to self and non-self antigens. Treg cells inhibit proliferative response and cytokine production by effector cells, basically via cell-cell contact. Deficiency or dysfunction in these cells may result in autoimmune diseases. Treg cells may also influence the outcome of infection, cancer, transplantation, allergy, and even some forms of infertility. Enhancement or blockade of natural Treg cells may represent a therapeutic approach to many immunopathologies. However, Treg cells act like a 'double-edged sword', which is why all therapeutic manipulation of these cells on humans should be done with great caution.