system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the actual difference between different blood groups?
gpt
Proteins on the surface of the blood cells. There are loads, but for the "normal" blood groups you think of, there are three different ones that are important: A, B, and one called "rhesus D".

If your blood has A proteins on the cell surface, your blood is type A. If B, type B. If it has both, type AB, and if it has neither, type O.

Separate to this, if your blood has the Rhesus D protein on its surface, you're a positive (+) blood group, and if it doesn't then you're a negative (-) blood group.