Membrane proteins involved in the activation and inhibition of the immune system through the interaction of antigen presenting cells and T-cells. B7-1 (orange), CTLA-4 (violet), MHC-II (red), CD4 (light blue), T-cell receptor (blue), CD-28 (dark blue), PD-L1 (yellow), PD-1 (violet), Interactions of MHC-II with the T-cell receptor and CD4 and B7-1 with CD-28 activates T-cells during the interactions of P7-1 with CTLA-4 and PD-L1 with PD - 1 deactivates T-cells.