system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do babies who are born to mothers with immunity to a disease not inherit that immunity?

For example if a mother was immunised against polio before pregnancy, and the baby is made up of her bodily materials, why does the baby still need to be immunised? Shouldn't it acquire the antibodies in her blood etc?

EDIT: Sorry, perhaps I shouldn't have used the word 'inherit' because that's more genetic. What I mean is more around the actual 'raw materials' that goes into making a baby. i.e. doesn't some of the mother's blood etc. make up part of the baby? Therefore aren't some of the antibodies passed on?
gpt
If antibodies remain high, there actually *is* some transfer through nursing. (Mammal secret weapon.)

But your immunity to something isn't the same as having high antibody levels all the time. That would be exhausting for your immune system and probably raises the risk of some kind of allergy or excessive immune response. 

So most of the time most of your immunities aren't actively producing antibodies. You have memory cells but no antigen, no antibodies. And for this reason antibody tests are better at showing that you are immune to an infectious disease *that you were recently exposed to.* After a few months, usually, the antibodies become much less common.

There's no placental immune system transfer (or from mother to egg) for a couple of reasons. But the big one is that would also be a good way to inherit parasites or cancer. Pretty much all complex organisms use reproduction as an opportunity to get a clean slate, no more parasites.

Many fish completely change their habitat when spawning, from salt to fresh water or vice versa. Like salmon for example. Ocean fish that spawns in fresh water. The adults die after spawning, which kills most parasites. Any surviving parasites (or eggs or cists) then find themselves in fresh water instead of salt. Bad news for them.

Finally, you probably don't want to remember immune patterns for too long because pathogens almost always evolve quickly. Imagine trying to keep 10,000 years of flu data! So resetting every generation is a convenient way to do that.