Development of Human Immune System in First and Second Trimester

Abstract

The immune paradox lies in the fact that the semi-allogenic embryo (or allogenic embryo in surrogate mothers) is not immunologically rejected [91]. A pregnancy may be spontaneously aborted in 10–20 % of cases without any apparent reason [2, 59]. There are multiple sets of immunological needs for the fetus and newborn, like protection against infection and bypassing harmful inflammatory immune responses that can lead to pre-term delivery, and also the transition from a sterile intra-uterine environment to external world full of unknown antigens. These immunological needs shape neonatal innate immune system dampening the production of pro-inflammatory cytokines rendering newborns at risk of infection and impairing responses to many vaccines [62]. Ontogeny of human fetal immune system in mid-term fetus is derived from a completely different set of stem cells than the adult immune system. Fetal immune system can tolerate antigens (including its mother and its own organs) better than to eliminate antigens from its environment. Midterm fetal immune system has about three times more regulatory T cells than newborns or adults, which render tolerance [73]. Treg blocked fetal T-cell responses to maternal cells.