Immunology of human implantation: from the invertebrates' point of view.

Pregnancies in outbred mammals may be regarded as successfully developed homografts. One of the hypotheses that attempts to answer the enigma of why the fetus is not rejected (fetal-maternal relationships are based on reciprocal expressions of foreign transplantation antigens) claims for the existence of possible evolutionary links between invertebrate allorecognition and mammalian implantation, based on some cellular similarities. This essay further discusses the possible evolutionary perspectives between vertebrates and invertebrates alloimmunities from a different viewpoint. We discuss similarities between natural transplantation in colonial marine invertebrates, which are followed by chimerism and a state of tolerance, and two natural transplantation events in the mammalian systems which both have records for prolonged chimerism and tolerance: the phenomenon of dizygotic twin fusions and the situation of fetal cells implantation. Earlier comparative evolutionary perspectives are revisited.