The release of cytokines is centrally important to many aspects of T cell function. IL-1 is involved in T cell activation. IL-2 is a potent T cell growth factor which is essential for the long-term proliferation of activated T cells. Equally, immuno-suppressive cytokines such as TGF-β and IL-10, are important for regulatory T cell actions. Helper T cells secrete many cytokine factors, including IL-4 and IL-5, which promote proliferation and differentiation of activated B cells. In addition, the migration of T cells occurs in response to chemo-attractant gradients provided by chemokines.