PD-1, also known as CD279, is an immune checkpoint receptor expressed on T and B lymphocytes following activation. Engagement of PD-1 by either of its ligands, PD-L1 (B7-H1) or PD-L2 (B7-DC), directly inhibits T cell receptor (TCR) signaling, leading to reduced cellular proliferation and cytokine production. TIGIT, also known as WUCAM and Vstm3, is an immune checkpoing receptor expressed on lymphocytes. TIGIT has several distinct mechanisms of action that inhibit lymphocyte activation. The PD-1+TIGIT Combination Bioassay reflects the mechanism of action (MOA) of biologics designed to block the PD-1/PD-L1 and TIGIT/CD155 interactions.