T-cell receptors, illustration. Array of immunologically active proteins and a calcium ion channel (dark purple, upper centre) on the membrane of a T-cell, a key part of the immune system. These proteins are involved in both the activation and deactivation of T-cells. The proteins are: TCR (blue), CD-4 (light blue), CD-28 (dark blue), PD-1 (pink), CTLA-4 (purple). CD-4 and CD-28 activate T-cells, while PD-1 and CTLA-4 inhibit the activation of T-cells.