Figure 4. Temporal evolution of keywords in immunotherapy research for RPL. This figure illustrates the temporal dynamics of keyword emergence in immunotherapy-related RPL research using a VOSviewer-based overlay visualization. The color gradient (blue to yellow) reflects the chronological appearance of keywords, with blue representing earlier research focus (1990s-2000s) and yellow indicating recent trends (2010s-2020s). Early phase (1990s-2000s): Emphasis on paternal cell immunization, leukocyte and lymphocyte immunotherapy, controlled trials, and antiphospholipid/anticardiolipin antibodies as early experimental approaches to immune modulation in RPL. Intermediate phase (2000s-2010s): Focus shifted toward immune regulation, particularly cytokines, NK cells, and their relevance in IVF and embryo transfer. Coagulation-related terms such as Factor V Leiden, thromboprophylaxis, heparin, and aspirin also gained prominence. Recent phase (2010s-2020s): Research has increasingly examined genetic and metabolic contributions to RPL, including vitamin D, cytokine gene polymorphisms, insulin resistance, and polycystic ovary syndrome. The rising use of meta-analyses and systematic reviews reflects a transition to evidence-based evaluation of immunotherapies. Topics like repeated implantation failure, tumor necrosis factor, and immune receptor expression suggest deeper exploration of molecular mechanisms underlying RPL.