PDCD1 and endometriosis: The reduction in the numbers and cytotoxicity of NK cells in the PF of endometriosis patients caused by the increased expression of inhibitory receptors, such as NKG2A, KIR2DL1 and LILRB1/2, immune checkpoint PD-1/PD-L1, TGF-β, IL-6, and IL-10, contribute to immune escape and ectopic endometrial tissue implantation.