Since the discovery of the first tumor phagocytosis-related checkpoint, namely the CD47/SIRPα axis, in the late 2000s, other tumor phagocytosis-related checkpoints have been identified: the PD-1/PD-L1 axis, MHC-I/LILRB1 axis, and CD24/SIGLEC-10 axis (Fig. 2). Here, PDCD1 is linked to neoplasm.