system
Given the following post, respond as if you're commenting on Reddit:
human
Why doesn't a woman's immune system attack the fetus during pregnancy?

Since the fetus has different DNA from the woman, why doesn't her immune system recognize the foreign body and try to kill it? When people receive new organs they have to take drugs to suppress their immune systems, why doesn't that happen with pregnancy?
gpt
Cells of the mother's immune system cannot cross the **placenta** into the baby. 

Only IgG anibodies can cross the placenta into the babies circulation, all other antibody classes are too big. The IgG which does cross the placenta is what gives the baby immunity during its first six months before its own immune system activates. 

Sometimes these IgG antibodies from the mother do attack the father's antigens in the baby's tissues. This is most commonly seen where a mother's antibodies attack a baby's red cells. The reason this does not happen more often is that many of the antigens in the baby's tissues are not fully developed and the mother's antibodies do not readily bind to them.