In some immune responses, a B cell or T cell becomes activated when an antigen or nonself cell binds to it. Activation then initiates proliferation. In most immune responses, however, activation requires the presence of a costimulator. Those two signals, an antigen and a costimulator, are required to initiate the immune response, ensuring that healthy self cells are not destroyed. Costimulation may occur in two ways: Cytokines, released by helper T cells and APCs, act as costimulators. Cytokines are protein hormones that influence cell growth. When a helper T cell becomes activated or an APC engulfs an antigen, the helper T cell or APC secretes a cytokine called interleukin. Helper T cells and APCs act as costimulators. Activated T cells or APCs that display antigens activate B cells or T cells when they temporarily bind to them.