Pregnancy is one of the most challenging experiences
for the immune system. It entails the confrontation and
cooperation of maternal cells with allogeneic (sperm)
and semi-allogeneic (fetal) cells and factors. On the one
hand, it must actively acquire a specific tolerance towards
the foreign cells and organism (the fetus) to avoid
harming reactions, while recognizing the very same to
support and control their development and growth. On
the other hand, the immune system simultaneously may
not reduce its capability to defend mother and fetus from
microorganisms and pathogens. Almost all branches of
the immune system are claimed to react and adapt in
order to fulfill these complex duties. In this review, current
knowledge concerning the most important cellular
and soluble immunological components in the decidua is
presented. Special regards are made to decidual NK, T
and dendritic cells as well as to trophoblast cells, representing
the fetal counterpart of most bilateral interactions.
Furthermore, the role and functions of soluble factors,
including HLA-G, PIBF, IDO and a variety of cytokines,
are described.