T cells need three signals to become fully activated. Signal 1 is provided by the T-cell receptor when recognising a specific antigen on a MHC molecule. Signal 2 comes from co-stimulatory receptors such as CD28, presented on the surface of other immune cells. It is expressed only when an infection was detected by the innate immune system, it is a "Danger indicating signal". This two-signal system makes sure that T cells only respond to harmful pathogens and not to self-antigens. An additional third signal is provided by cytokines, which regulate the differentiation of T cells into different subsets of effector T cells.