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A third class of T cells called regulatory T cells (Tregs) ensures that the immune system does not attack self cells and molecules indiscriminately. Like T_H and T_C cells, Tregs mature in the thymus gland, carry T cell receptors, and become activated if they bind to antigen–MHC complexes. But Tregs are different in one important way: the antigens that Tregs recognize are self antigens. The activation of Tregs causes them to secrete cytokines, which blocks the activation of T cells that are bound to the same antigen-presenting cell (Figure 41.14). Thus Tregs constitute another mechanism for distinguishing self from nonself.