A mother's immune system can learn to attack her own child, and it learns it from the baby before.
Some people carry a protein on their red cells called Rh; some don't. If an Rh-negative mother carries an Rh-positive baby, their blood usually mingles a little at birth. Her body sees cells it doesn't recognise and does what immune systems do: it remembers.
The first baby is almost always fine. The danger is to the next one. Those antibodies cross the placenta and attack the baby's red cells. Before treatment existed, Rh disease killed or damaged a great many infants — in severe cases most did not survive.
The fix, licensed in 1968, is beautifully backwards. Ready-made anti-D antibodies are injected into the mother, and they clear the baby's stray cells away before her own immune system ever gets to study them.
A vaccine works by creating a memory. This one works by preventing one.