PDCD1 and cancer: NK, natural killer; DC, dendritic cell; Treg, regulatory T cell; Th17, T helper type 17; MDSC, myeloid‐derived suppressor cell; CAF, cancer‐associated fibroblast; TAM, tumor‐associated macrophage; exo, exosome; PD‐1, programmed cell death protein 1; VEGF, vascular endothelial growth factor; FasL, Fas ligand.