Activation of T-cell immune response, illustration. Array of proteins interacting on the surface membranes of a T-cell (across bottom) and an antigen presenting cell (APC, across top). Shown here is the interaction of MHC-II (red) with the T-cell receptor (TCR, blue), and CD4 (light blue) and B7-1 (orange) with CD-28 (dark blue), activating T-cells. The interaction of B7-1 (orange) with CTLA-4 (purple) and PD-L1 (yellow) with PD-1 (purple) deactivates T-cells. T-cells are a key part of the immune system. For this illustration with full labelling, see image F023/0923.