Specifically wondering why surrogate’s body doesn’t attack the zygote/fetus as if it’s a foreign body.
Of course as I’ve typed this out I’m wondering if it’s the same reason babies aren’t attacked in the womb in any other situation, the placenta?
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Baby’s can be attacked. In fact if the baby is RH positive, and the person who’s uterus that they are growing in, is not RH positive, antibodies will be created and attack the baby.
In this case the doctors will have to administer medicine to stop this from happening.
That being said, your body is not testing the DNA of every cell that comes in, to attack it if it is foreign. It is mostly reacting to things that it knows will cause trouble. That is why new viruses and the like, can cause so much havoc as the body will allow it to gain a pretty strong foothold before it decides to spend the energy to kick it out.
Same question goes for any pregnancy, actually. Not counting mitochondrial DNA, 50% of your genetic material comes from your father, which is foreign to your mother’s body. What’s to stop a pregnant woman’s immune system from attacking her gestating fetus? We don’t really know all the details. The hand-wavy incomplete answer is that the mothers immune system is suppressed by hormones from the placenta. Exceptions exist.
>Of course as I’ve typed this out I’m wondering if it’s the same reason babies aren’t attacked in the womb in any other situation, the placenta?
Spot on. The unborn child is *always* foreign, even in a perfectly normal pregnancy. The reason it’s not a problem most of the time is that the placenta is a very restrictive barrier that only tiny molecules (notably, oxygen) can naturally diffuse across. Cells and the vast majority of molecules that make up the maternal immune system don’t get to cross.
Antibodies of the IgG isotype do get shuttled across, and that can be problematic when, following a bleed that allows blood contact between mother and child, the mother gets sensitized against the child’s blood group antigens (notably RhD) or HLA.
I asked a lot of questions about the science behind it, I’m a surrogate and a curious person. My doctor said that my DNA and the baby’s DNA never interact. I was told that the placenta filters everything (fun fact, the placenta is made almost entirely with the male’s DNA) I got very simplified answers so I’m sure that there’s more too it but it’s crazy when you think about the science behind it all.
What you don’t know is that the placenta has a membrane through which oxygen and nutrients pass from the mother’s blood to the fetus’s and CO2 gets passed back. The mother’s blood does not mix with the fetus’s blood and so the mother’s immune system does not normally sample the fetus’s proteins to start an immune reaction.
Other than that the mother’s immune system is partially inactivated starting from the implantation. [Science Daily article](https://www.sciencedaily.com/releases/2012/06/120607142244.htm)
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