T cells with the specialized ability to suppress both adaptive and innate immune responses have been identified and called T regulatory cells (Tregs). The primary function of Tregs is to maintain a balance between immunity (foreign Ag) and tolerance (self Ag) to tissues. Tregs prevent autoimmune disease, maintain immune homeostasis and modulate protective responses against infection. Tregs function in two ways; 1) limiting the magnitude of effector responses which influence the adequate control of infection and 2) control collateral tissue damage caused by vigorous antimicrobial responses against pathogens. Initially, the immune suppressive ability of CD4 T cells was predicted by expression of the forkhead box p3 (Foxp3) transcription factor. However, many reports have demonstrated immune suppressive function in an array of other T cells which include iT(R)35, CD8+, NKT cells, especially in mucosal tissues. The immune suppressive mechanisms of Tregs include contact-dependent, cytokine secretion and regulation of immune cell migration. The expanded group of Tregs is crucial for protecting the function of mucosal tissues such as the gut, respiratory and genital tracts, as these tissues are routinely exposed to foreign pathogens.