Pregnancy is a period which requires a suppression of mother’s immune system in the uterus. This allows the fetus to implant itself and enables its unobstructed development. Some of the changes in the immune system that enable this process consist of the establishment of local immunotolerance of T cells, a reduction or loss of cytotoxicity in NK cells, a polarization of macrophages after trophoblast invasion and a reduction in activating signaling molecules in APCs. Besides the local immunotolerance, a global immunotolerance has been discovered, which occurs due to tolerogenic T cells spreading throughout a pregnant woman’s body. This tolerogenicity expansion leads to a reduction in the symptoms of autoimmune diseases not connected to pregnancy, such as the Graves’ disease, however, it does not make the pregnant woman susceptible to all infections, but only to certain pathogens such as Lysteria monocytogenes. There is a host of factors that influence the behavior of decidual immune cells, the key factor being mother’s diet, and in recent years, vaccination is thought to also have an important role. Mother’s immune system regulates the development of the immune system of the fetus and modulates the neonatal immune response during the pregnancy. IgG has a primary role in these processes. It passes through the placenta and stays in the newborn’s body for six months, which enables passive immunity. Another type of transfer of immune cells and other soluble immune factors is carried out through mother’s milk, which is why breastfeeding is recommended instead of using artificial milk substitutions.