PDCD1 and lung cancer: Furthermore, CD4+ helper T cells and CD8+ cytotoxic T cells present in lung cancer contribute to the production of immune exhaustion proteins, including cytotoxic T-lymphocyte antigen 4 (CTLA-4), lymphocyte activated gene 3 (LAG3), programmed cell death protein 1 (PD-1), etc., leading to tolerance of tumor antigens and loss of anti-cancer effects in these cells [13].