system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why does the body reject transplanted organs?

What is it about a transplanted organ that tells the body it's foreign and doesn't belong?
gpt
Every cell in your body is covered with "Human leukocyte antigens" which is a crazy random protein that is different for basically everyone alive.  The immune system looks for all sorts of things, but if it finds cells and they don't have the right HLA then it defaults to "kill this thing".