Certain sensors in the mother's immune system are desensitised to make it possible for her body to accept the 'alien' within: her child
IT HAS long been an enduring enigma as to how a foetus, genetically very much a 'foreign' entity, is tolerated by the mother's immune system through the months of her pregnancy. At the molecular level it is rather puzzling that the foetus manages to retain a foothold even though many of the proteins that it expresses are encoded by genes derived from the father -just the sort of proteins that the mother's immune network is designed to hunt out and destroy ( Current Biology, Vol 6, No 1).
Indeed, recent research does suggest that a foetus is born against consider- able odds because the mother keeps try- ing all the while to get rid of it. The enormous problems faced by surgeons who attempt to transplant tissues or organs, serve as testimony to the efficacy of the immune system in other situa- tions where the body encounters these alien bodies.
A recent article by Elizabeth Simpson of the Hammersmith Hospital in London, UK, discusses the issue of maternal-foetal compatibility. It turns out that a possible solution to the mys- tery lies in the behaviour of a $pecific subset of cells in the mother's immune system, known as T cells or T lympho- cytes. These are the main mediators of the 'host-versus-graft' reaction. These are the cells that make transplantation so difficult. T lymphocytes contain a number of molecular sensors on their surface which are capable of detecting paternal antigens. With the onset of pregnancy, the number of sensors are reduced so that the sensitivity of the immune response to foreign proteins encoded by the father's genes, is specifically lowered. The important point to note is that the reduction of sensitivity is very pointed and specific. There is no impairment of the strength of the immune response to other antigens. Also, the permission given to the father of the child to invade the mother's system, is time-bound and concomitant with the period of pregnancy.
Experiments have shown that the process of adaptation to the father's antigens is similar to the process of self - recognition that the foetus's own immune system has to undergo (it is of critical importance for the immune system not to try and destroy the body within which it resides). This is suggested by the striking obser- vation that patients suffering from auto-immune diseases appear to go through a period of remission during pregnancy.
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