The Role of Immunological Bystander Reaction in Recurrent Pregnancy Loss

Recurrent pregnancy loss (RPL) affects approximately 1%-2% of women of reproductive age and may be associated with immunological bystander reactions. These reactions occur when the maternal immune system's response to a specific target extends to nearby tissues, causing collateral damage to both maternal and fetal cells. This narrative review explores the mechanisms of immune dysregulation at the maternal-fetal interface and evaluates current therapeutic strategies. A literature search for a narrative review was conducted using PubMed, Scopus, and Web of Science for the period 2010-2025. Keywords included "immunological bystander reaction," "recurrent pregnancy loss," and "immune therapy in pregnancy". Following a Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA)-based screening process, 41 reports were selected for final analysis. A successful pregnancy requires the maternal immune system to maintain a delicate balance between protecting against pathogens and tolerating the semi-allogeneic fetus. Bystander activation, often mediated by soluble cytokines, allows non-antigen-specific T cells to proliferate and cause tissue damage without prior sensitization. A critical factor is the Th1/Th2 cytokine balance: pro-inflammatory Th1 cytokines (e.g., tumor necrosis factor alpha (TNF-α), interferon gamma (IFN-γ)) are associated with fetal resorption, while Th2 cytokines (e.g., interleukin (IL)-4, IL-10) promote immune tolerance. The review highlights a significant shift in clinical understanding, particularly the work of Professor Hassan Shehata, who challenges the traditional focus on thrombophilia. Data shows that the prevalence of inherited thrombophilia in RPL patients (8.1%) is nearly identical to the general population, suggesting that "sticky blood" is not the primary driver in most cases. Instead, the focus is moving toward reproductive immunology, specifically the role of overactive natural killer (NK) cells and cytokine imbalances. According to the 2025 American Society for Reproductive Immunology (ASRI) guidelines, management is shifting toward precision medicine. Prednisolone is recommended for women with documented immune abnormalities (e.g., uterine NK (uNK) cells >5% or antinuclear antibody (ANA) positive) to suppress inflammation and promote Th2 dominance. Hydroxychloroquine (HCQ) is conditionally recommended for established autoimmune diseases or specific inflammatory placental pathologies. Tacrolimus, a granulocyte-colony stimulating factor (G-CSF), and intralipids are currently categorized as having unclear benefits due to a lack of robust randomized controlled trial evidence. Maternal inflammation is a primary trigger for disturbed placentation and fetal loss. Improving outcomes for women with RPL requires a proactive, evidence-based approach that specifically targets the immunological bystander reaction pathway rather than relying on routine clotting tests.