Placenta - A Competent, But Not Infallible, Antiviral and Antiparasitic Barrier.

Significant challenges have been faced in the last decade due to viral and parasitic infections that have significantly impacted human health. Following an outbreak of the Zika virus, which has been linked to neurological issues in both adults and developing fetuses, we have recently dealt with SARS-CoV-2 infection, which has rapidly spread worldwide. Parasitic infections such as Malaria, Toxoplasmosis, and Chagas Disease severely affect the health of underresourced communities. In this scenario, the possibility of vertical transmission and fetal involvement is of significant concern. For protection, the fetus relies on the maternal immune system and placenta during intrauterine development. As an interposed organ between the mother and baby, the placenta performs numerous vital functions in fetal development, including multiple defense mechanisms against different types and intensities of stressors such as viral and parasitic infections. The role of the placenta in preventing pathogen access to the fetus and the factors involved in the failure of this mechanism need to be completely elucidated. In this scenario, syncytiotrophoblast plays a central role, using several strategies to prevent viral replication. Recent advances in the understanding of the molecular basis of these mechanisms include interferon (IFN) type I and III signaling, autophagy regulatory microRNAs, and the activation of the NF-κB pathway, which opens the possibility of cytokines and chemokines expression with local and systemic actions in the maternal organism. In this review, we address the role of the maternal-fetal barrier, chorionic villi in contact with maternal blood, antiviral and antiparasitic defense mechanisms, and mechanisms used by the human placenta against some of the most common viral and parasitic infections. Many of these strategies have successfully prevented congenital infections or fetal damage.