Non-small cell lung cancer (NSCLC) is the most common type of lung cancer. There are several studies which demonstrate a role for the immune system in fighting lung cancer. However, there are multiple mechanisms by which cancer dampens this response. The PD-1 receptor-ligand interaction is one of the major pathways hijacked by tumours to help evade detection and elimination by the cells of the immune system. A number of compounds which block this pathway, including the drug pembrolizumab, have shown impressive results in some patients.