1 Introduction Regulatory T cells (Tregs) are understood as the main key players in maintaining peripheral immune tolerance. After their initial description (1, 2), there has been growing knowledge around Treg subsets and their subset-specific mechanisms of action. A milestone in understanding Tregs was the discovery of the function of forkhead box P3 (FOXP3) (3, 4). In clinical practice, Treg subsets are not routinely analyzed in patients with immune dysregulation.
