Table 1.

Systematized search using the PICOS method.

Description	Abbreviation	Question Components
Population	P	Women who used ARTs had RAR, RIF, and IVF failure.
Intervention	I	Performance of blood collection or endometrial biopsy for dosage of immune cells and/or inflammatory cytokines before and/or after embryo transfer.
Comparison	C	Fertile women who achieved clinical pregnancy.
Outcome	O	Obtained key IS cells and/or secreted cytokines found in peripheral blood or from endometrial cells.
Study	S	Observational and case-control studies.

ART: Assisted Reproductive Techniques; IVF: In vitro fertilization; RAR: Recurrent Pregnancy Loss; RIF: Recurrent Implantation Failure; IS: Immune System.