Mechanism of action of immune checkpoint inhibitors. Left: the priming phase of the T cell is shown. Antigen-presenting cells (APC) activate a naïve T cell by presenting a tumor antigen using their major histocompatibility complex (HMC), which is recognized by the T cell receptor (TCR). Possible inhibitory signals are blocked by anti-PD-L1, anti-PD-1, and anti-CTLA-4. Right: the activated T cell infiltrates the tumor tissue. There it recognizes the tumor antigen, which initiates an immune attack. Possible inhibitory signals are blocked by ant-PD-1 and anti-PD-L1.