system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come Type O- (O negative. . . ) can be given to most other blood types but it can only receive it's own blood type?
gpt
When we talk about blood type, we're really talking about the different proteins in your blood. The biggest ones are the A protein, the B protein, and the Rh factor. If your body doesn't have the protein and you're given blood with the protein in it, your immune system will attack it. Someone with O- blood has none of these proteins, so if they get blood from someone with any of the three (either A, B, or +), their immune system will go crazy. 

Getting blood that doesn't have a protein that you naturally have is fine, which is why someone with O- blood can donate to anyone.