Immunologically active proteins on a T-cell. TCR (blue), CD-4 (light blue), CD-28 (dark blue), PD-1 (magenta), CTLA-4 (violet), Ca-channel (dark violet). The T-cell receptor, CD-4 and CD-28 activate T-cells, while PD-1 and CTLA-4 inhibit the activation of T-cells.