ELI5; How does Penicillin work?

Question

I understand a vaccine, a small dose of ‘x’ is given so your body can realise it’s foreign and learn how to fight it when the real thing comes.
But surely with a penicillin dose not being a vaccine,the body now has both it AND the foreign body to deal with? How does it’kill’ the intruder and not detrimental to the immune system?

Edit; Added words for clarification.

Edit 2; Thanks all, I have an understanding now!

In: 6

Answer 1

Penicillin isn’t a vaccine, it’s an antibiotic.

It kills bacteria in your body, then a few hours later you pee it out.