Blood type

Lily • My gorgeous little boy due 8th of March 2018💙

So I'm 10 weeks tomorrow!!! Just had a letter through saying that my blood group is A negative, I'm unsure on my partners. I've read if mine and baby's blood get mixed my body could fight off the baby? I know I've to go and get an injection when I get to 28 weeks and after birth, but does anyone know more about it?