Why does a mother's body not reject the "foreign body" aka fetus that is developing inside of her?

Question

I have a question of my homework that asks "The human body's immune system normally tries to get rid of any foreign matter it encounters ie. bacterial infections. This is also why organ transplants are sometimes rejected.

What prevents a mother's immune system from rejecting the fetus developing inside of her?

I have absolutely no idea so any help would be appreciated.

Thank you!

Thank you so much guys! I really didn't know about the immunosuppressants and stuff!

I'm glad there's scientists and doctors and professors on this website to explain this stuff to us high school students!

Answer 1

Great question! The fetus is, indeed, an "allograft", meaning that the body should recognize it as foreign and thus go into defensive mode. This is a topic that scientists are still wrestling...

However, there are a few studies that point us in the right direction.

First remember that there is no direct interaction between the fetus and the mother in terms of cell exchange, (despite what is mentioned below) particularly in the blood -- all material exchange including waste, oxygen and nutrients is passed through the placenta, (exactly like what happens in the lungs with oxygen and carbon dioxide) which is made up partly of the fetus' DNA and partly of the mom's DNA. This makes the placental wall 75% mom; meaning that it is only marginally seen as an allograft -- maybe an "allograft-light." :)

Now for the other side: Progesterone is a hormone produced by all female mammals when preggers. It has been found that progesterone is itself produced in parts of placental tissue. (Most of it is produced elsewhere in the mom.) It is not surprising, then, that progesterone has been found to be a mild immunosuppressant in rats. What's more, the placenta ALSO produces a bit of its own immunosuppressant called IDO.

So, it would seem that evolution has met 1/2 way here -- the placental sac tries to be as much like the mom as possible so that allograft-detecting sensors in the mom remain marginally clueless, and those that ARE smart enough to figure it out are dramatically weakened by progesterone and IDO.

Regarding an above answer: Actually, the female body IS programmed to reject sperm -- that's why it takes so many to induce pregnancy. It is also not true that the human body only attacks bacterial invasions. If that were the case, no one would have seasonal allergies, and we would have all died out from the flu 10's of thousands of years ago.

Answer 2

the fetus is absolutely a foreign body -- half of its genes came from a different person. maternal immunology is not very well understood. however, there are some thoughts. one of the main reasons for the lack of rejection is a physical barrier that restricts the movement of T cells into the fetus. the placenta allows for red blood cells and nutrients to pass, but not larger cells, like lymphocytes. thus, the immune system has a hard time "seeing" the fetus. secondly, the mother's immune system is largely impacted by the hormonal milieu associated with the fetus, both locally and systemically. evidence of this is that autoimmune diseases will typically wane during pregnancy and then return after the birth of the child. thus, the immune systems seems to be quelled, or altered in some way, during the gestation period, and this may help lead to overall acceptance of the fetal "graft." several other ideas are out there, but those are some of the basic ones.

Answer 3

Sometimes a mother's body does reject the fetus. I've forgotten what the condition is called. Good luck. 2D

Answer 4

The fetus is not a foreign body, and definitely not bacterial. The woman's body is also programmed to not reject semen, but otherwise everything about the developing fetus is her own body and we don't reject cells from our own bodies.

Answer 5

You'd better give cantonrh the best answer... he is right on and very well said I might add!

Answer 6

Simple..they are the same body.At least until she 'drops' it.