 Blockade of CD28 is effective in stopping T cell activation, a mechanism that the immune system uses to down-regulate T cell activation.T cells can express the surface protein CTLA-4 (CD152) as well, which can also bind B7, but with twenty times greater affinity for B7 proteins, and lacks the ability to activate T cells.As a result, the T cell is blocked from receiving the B7 protein signal and is not activated.