The main function of our immune system is to protect us from invading pathogens and microorganisms by
destroying infected cells, while minimizing collateral damage to tissues. In order to maintain this balance between
immunity and tolerance, current understanding of the immune system attributes a major role to regulatory T cells
(Tregs) in controlling both immunity and tolerance. Various subsets of Tregs have been identified based on their
expression of cell surface markers, production of cytokines, and mechanisms of action. In brief, naturally occurring
thymic-derived CD4+CD25+ Tregs are characterized by constitutive expression of the transcription factor FOXP3, while
antigen-induced or adaptive Tregs are mainly identified by expression of immunosuppressive cytokines
(interleukin-10 (IL-10) and/or transforming growth factor-β (TGF-β)). While Tregs in normal conditions regulate
ongoing immune responses and prevent autoimmunity, imbalanced function or number of these Tregs, either
enhanced or decreased, might lead, respectively, to decreased immunity (e.g., with tumor development or infections)
or autoimmunity (e.g., multiple sclerosis). This review will discuss recent research towards a better understanding of the
biology of Tregs, their interaction with other immune effector cells, such as dendritic cells, and possible interventions in
human disease.