Co-stimulation

An event in the immune system involving the delivery of a second signal by an antigen-presenting cell.

The second signal rescues the activated T cell from anergy (which is a state of immune unresponsiveness), allowing the T cell to produce the lymphokines necessary for the growth of additional T cells.</P> T cells are small white blood cells that orchestrate and directly participate in the immune defenses.

Also known as T lymphocytes, they are processed in the thymus (which is why they are called T cells).