Newborn infants are susceptible to a serious infection by a wide range of pathogens, especially infants who are born prematurely. This has generally been interpreted to mean that the developing immune system is inactive, ineffective, or uneducated. To the contrary, the human fetal immune system is highly functional, but its primary function appears to be establishing and maintaining immune tolerance.
Human fetal immune development proceeds in such a manner that a dominant tolerogenic fetal immune system is progressively replaced by an immunogenic adult-type immune system, resulting in co-existing ‘layers’ of cell populations.