Figure 1.

Major mechanisms of action of monoclonal and bispecific antibodies. Unconjugated IgG1 monoclonal antibodies (MAbs) work generally through activation of immune effector mechanisms through their Fc regions: (A) Antibody-dependent cellular cytotoxicity (ADCC) by NK cells and antibody-depndent phagocytosis (ADCP) by macrophages, (B) activation of the complement cascade. (C) T cell engaging bispecific antibodies (BsAbs) with or without Fc, act by binding a tumor antigen (TA) and CD3 on T cells (CD3 x TA). This induces activation of cytotoxic T cells which proliferate and kill the tumor cells. (D) Antibodies against immune checkpoint inhibitors (ICI) mostly block interaction of the ICI with their ligands, thus activating immune cells. This takes place via the Fab interaction with the ligand, blocking ICI function. In some cases, the MAbs may have a functional IgG1 Fc and eliminate ICI expressing cells through ADCC/ADCP or complement-dependent cytotoxicity (CDC). See also Table 1 for abbreviations.