system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:I have O-Negative blood, I can donate to anyone in the world, but only another O-Negative can donate to me in emergency. Why?
gpt
Your blood is special because it lacks any of the common markers which would make a body unused to them react with force from the immune system.  The downside is that your body is accustomed to blood without any of these markers, and will identify blood with any of them as foreign and react accordingly.