Regulatory T cells (also called Tregs) are T cells which have a role in regulating or suppressing other cells in the immune system. Tregs control the immune response to self and foreign particles (antigens) and help prevent autoimmune disease.
Natural Treg are characterized as expressing both the CD4 T cell co-receptor and CD25, which is a component of the IL-2 receptor. Treg are thus CD4+ CD25+. Expression of the nuclear transcription factor Forkhead box P3 (FoxP3) is the defining property which determines natural Treg development and function.
FoxP3 is crucial for maintaining suppression of the immune system. Naturally occurring mutations in the FOXP3 gene can result in self-reactive lymphocytes that cause a rare but severe disease IPEX (Immune Dysregulation, Polyendocrinopathy, Enteropathy, X-Linked) in humans.
(Source: British Society for Immunology)