Co-stimulation

Co-stimulation is a secondary signal which immune cells rely on to activate an immune response in the presence of an antigen-presenting cell. In the case of T cells, two stimuli are required to fully activate their immune response. During the activation of lymphocytes, co-stimulation is often crucial to the development of an effective immune response. Co-stimulation is required in addition to the antigen-specific signal from their antigen receptors.

== T cell co-stimulation == T cells require two signals to become fully activated. A first signal, which is antigen-specific, is provided through the T cell receptor (TCR) which interacts with peptide-MHC molecules on the membrane of an antigen presenting cell (APC). A second signal, the co-stimulatory signal, is antigen nonspecific and is provided by the interaction between co-stimulatory molecules expressed on the membrane of the APC and the T cell. This interaction promotes and enhances the TCR signaling, but can also be bi-directional. The co-stimulatory signal is necessary for T cell proliferation, differentiation and survival. Activation of T cells without co-stimulation may lead to the unresponsiveness of the T cell (also called anergy), apoptosis or the acquisition of the immune tolerance.