Question

fetus as an Allograft but not occur rejection ?why

Answer 1

The exact mechanism through which mother and fetus co-exist during pregnancy has been a subject of extensive scientific research and debate. It appears this remarkable state has been achieved through multiple adjustments, amounting to a localised down-regulation of the normal immune reaction allowing the fetus to flourish unperturbed. Below are just some of those:

1. There is evidence that the layer of trophoblast cells of the placenta secrete specific proteins into the mother’s circulation. These proteins have the effect of inhibiting her Natural Killer (NK) cells and cytotoxic T lymphocytes, which are normally responsible for attacking and destroying the foreign tissue.

This is believed to create an inert environment at the site where the normal immune reaction does not take place. This effect is strictly local, leaving the woman’s normal general immune capabilities intact.

2. Progesterone is a hormone that is known to be crucial both for preparing the womb environment for implantation after conception has taken place and for the maintenance of pregnancy. Initially the progesterone is sourced from the cyst left behind after ovulation.

The cyst goes by the name corpus luteum. A few weeks down the line, the placenta takes over the production of this important hormone. It transpires that progesterone produced by placental cells is crucial in preventing fetal rejection as it actively inhibits T-cell mediated foreign tissue rejection in the womb.

3. An enzyme known as IDO (Indoleamine 2,3-dioxygenase) is found in much higher concentrations in the pregnant uterus. It turns out that it is actually produced by the fetus.

IDO is responsible for breaking down the amino acid Tryptophan. This action has the effect of inhibiting the proliferation of the T cells which would otherwise be cytotoxic to the foreign tissue that is the fetus.