Why do bacteria and blood cells usually not cross the placental barrier?

The placental barrier serves as a selective interface between maternal and fetal circulations. It is composed of the fetal blood vessel walls, the connective tissue of the villous core, and the syncytiotrophoblast layer.1

The reason bacteria and blood cells generally do not cross this barrier is due to the structural integrity of these layers, which maintain a separation between the two distinct bloodstreams. While the barrier is permeable to essential substances like nutrients, water, oxygen, and certain hormones, it acts as a physical filter that prevents the passage of larger cellular structures, such as maternal or fetal blood cells, and most microorganisms.123

It is important to note that this barrier is not absolute; while it effectively blocks many pathogens, it can be crossed by viruses and certain drugs. Furthermore, the permeability of the barrier varies across different species, and in humans, it specifically allows for the transfer of maternal IgG antibodies to provide the fetus with passive immunity.1342