system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What's in the middle of a canker sore?

There's some white stuff in the middle of a canker sore, surrounded by a dark red ring.  What's the white stuff?  A staph infection?  White blood cells?  Dead cells?  Thanks!
gpt
The yellow-white stuff is known as a "fibrin membrane". Fibrin is a blood protein that aids in scab formation, and it will ooze out of shallow wounds easier than red blood cells. Have you ever seen certain road rashes heal? The countless scratches can be big enough to let fibrin flow, but too small to let much whole blood out, resulting in yellow-white scabs.