It even sounds really ridiculous and not worthy to compare a human embryo with a parasite. But since lately some ideological movements have call the embryo a “parasite” which lives at the expense of the woman’s body, let’s clarify the issue.
What the science has to say
We all know that white cells are a special type of blood cells which protect us from infections (bacteria, virus and parasites). If the embryo would be just a parasite, the woman’s immune system would attack and reject it as it does against any parasite. But, what happens instead?
The first day after implantation, there is a special kind of white cells rapidly recruited to the uterus and activated: “Tregs”. This white cells are regulatory immune cells, which allow for the embryo not being rejected by the maternal immune system, despite being genetically different from her because the paternal genes present in the embryo’s body.This mechanism has been deeply studied in a very detailed way by multiple scientists around the globe.
Not only these cells protect the fetus during pregnancy, but rather the uterus cyclically prepares itself for pregnancy accumulating these protective cells each month following the hormonal changes associated to the female period. Even more, these regulatory cells are temporarily recruited to the uterus after intercourse, just in case it could lead to pregnancy.
We can also think in the cases when some patients need an organ transplant; a medical intervention is necessary to suppress the immune system so the received organ is not rejected. This highlights how no medical intervention is needed on the woman’s immune system to produce this maternal-fetal tolerance, because the embryo is not a “strange organ”, but someone whom the mother’s body is prepared to receive.
To summarize, completely different to what happens upon a parasitic infection, during pregnancy the human immune system mounts a complex mechanism which prepares the uterus under hormonal control, ensures the maintenance of immune tolerance to the fetus and allows “vascularization to guarantee a steady supply of nutrients and oxygen to the fetus for a proper growth and development” (Ruocco, 2014).