system
Given the following post, respond as if you're commenting on Reddit:
human
How come vaccines don’t pass from mother to child?

If they share the same blood before birth, which I’m 90% sure they do, wouldn’t their immune system keep information from when the mother got vaccinated?
gpt
The mother and the fetus do not share the same blood. The function of the placenta is to facilitate the exchange of nutrients, gases, and wastes while keeping the blood supplies separate. There are a type of antibodies called IgG that can cross the placenta. They give some protection to the developing fetus in utero. After birth, IgG and IgA antibodies are transferred to the baby through breast milk. While the baby is breastfeeding, it has the same immunities as the mother does (including those the mother acquired through vaccination). Babies start to make their own antibodies by three months, but their immune systems will be supplemented by maternal antibodies until they are weaned. After weaning, maternal antibodies in the baby's blood eventually degrade, so they will have to be vaccinated to develop their own antibodies.