system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What exactly is the blood-brain barrier?
gpt
As a concept, it's the idea that your body seriously restricts what can get from your bloodstream to your brain and spinal cord (which are bathed in cerebrospinal fluid.) The brain is delicate and needs to be protected from stuff that will mess with its function, and in particular from infection. Infection in the brain is very bad news, as both the infection and your immune response can damage things.

Practically, the blood-brain barrier is mostly made up of tight junctions between cells that limit what gets through, plus pumps that get rid of many things that do get in. 

(As you may have gathered from the existence of medications and street drugs that do affect the brain, it's still not 100%.)