Definition of Co-stimulation

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.

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).