PDCD1 and neoplasm: Based on their potential functional impact, neoantigens can also be divided into protective neoantigens that are cross-recognized by pre-formed memory T cells induced by heterologous immunity, restricted neoantigens that are immunogenic in immunotherapy hosts and induce PD-1+ memory T cells, and neglected neoantigens that do not induce associated immune responses in tumor-containing hosts but can effectively promote tumor immunity once memory T cells are induced by vaccination [41].