A mother's immune system may attack a fetus from multiple fronts, by sending in killer T cells, by producing antibodies that target fetal cells, or by coating cells with a destructive, soluble blood factor called complement, according to researchers at the Medical College of Georgia. The grim attack described by researchers may surprise immunologists who would expect the T-cell attack, but not complement, the lethal component of an immune response that more typically would attack invaders such as viruses and bacteria coated by antibodies made by B cells. We assumed that the maternal T cells would get activated by fetal antigens, expand in number, then start to kill fetal cells and we cannot exclude that at this point. But in research published in the January 2001 issue of Nature Immunology , they also found deposits of complement - a component of the innate immune system which neutralizes foreign objects such as invading bacteria or virus coated by antibodies. In a normal immune response, T cells come in to attack and destroy viral-infected cells while antibodies and complement attack bacteria and viruses still floating around in the bloodstream.
Antibodies coat viral cells, activating the complement deposition and eventually killing the virus.
The broad assumption has been that there is not a connection between T cells and complement. To further explore new evidence that there may be, the MCG researchers already are looking at a model that has a factor produced by T cells which may activate the complement. The MCG research team, led by Mellor and Dr. David Munn, described in the August 1998 edition of Science a mechanism for localized suppression of the mother's immune system that enables the fetus to escape attack. They found that during pregnancy, cells in the placenta trigger an isolated suppression of the mother's immune system so it doesn't reject the genetically foreign fetus. IDO disables the immune system by degrading tryptophan, an amino acid essential to the survival of T cells.