Figure 1. A proposed algorithm for achieving effective antitumor immune responses. (A) The blockade of the programmed death 1 (PD1)/PD-L1 pathway may be effective against tumors in which PD-L1 is expressed at the cell surface, reflecting a baseline, “pre-activated” state of the immune system. (B) The therapy of tumors in which the immune system is not in such a “pre-activated” state may require additional interventions to generate an inflammatory microenvironment, hence priming immune effector cells for mediating antineoplastic functions once the immune checkpoints are relieved (for instance with anti-PD1 antibodies).