Comment by ufo

IIRC, in this particular case the antibodies bind to immune cells. Immune Checkpoints are a mechanism that keeps the immune system from attacking the own body but in cancer it can also stop the immune system from destroying the cancer. The checkpoint inhibitor antibodies remove these restrictions and allow the immune cells to attack the cancer. (The price is that they also become free to attack other things they shouldn't; autoimmune inflamations are common side effects.)

https://en.wikipedia.org/wiki/Checkpoint_inhibitor