system
Given the following post, respond as if you're commenting on Reddit:
human
How does Cancer cross the blood brain barrier?
gpt
The blood brain barrier prevents chemicals from crossing from the blood stream into neurons. Circulating Cancer cells will just edge themselves in a capillary and start driving there forming a ball of tissue .

It is like you putting a fence to prevent rabbits from entering your backyard and eating your plants, but then you get a bunch of squatters sitting up camp on the fence, eventually braking it.