Expression of complement regulatory proteins on fetal blood cells in utero.

Since the fetus is semiallogenic to the mother, mechanisms have evolved to protect fetal tissue from the maternal immune response. Among these mechanisms is the expression of cell-surface complement regulatory proteins at the maternal-fetal interface. However, beginning in the third trimester, fetal blood cells are exposed to actively-transported IgG antibody. Thus, we speculated that fetal blood cells would require expression of one or more complement regulators by the early third trimester. Using flow cytometry and Western blots, we have demonstrated the presence of three important complement regulatory proteins in the circulating blood cells of human fetuses. These findings are consistent with the putative biological role of the cell-surface complement regulatory proteins.