Regulatory T cell
Regulatory T cells sometimes known as suppressor T cells, are a specialized subpopulation of T cells which suppresses activation of the immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

 and thereby maintains tolerance to self-antigens
Immune tolerance
Immune tolerance or immunological tolerance is the process by which the immune system does not attack an antigen. It can be either 'natural' or 'self tolerance', in which the body does not mount an immune response to self antigens, or 'induced tolerance', in which tolerance to external antigens can...

. The existence of regulatory T cells was the subject of significant controversy among immunologists for many years. However, advances in the molecular characterization of this cell population have firmly established their existence and their critical role in the vertebrate immune system.