system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What are the differences between blood types?
gpt
There are slight differences in red blood cells that determine blood types. If you have certain differences, then your immune system learns when you're an infant not to attack those cells because they're not foreign invaders. But if you don't have them, and you get a blood transfusion from someone else who does, then your immune system that's never been exposed to those will think it's a foreign invader and attack it, which will result in a dangerous and possibly deadly reaction.

Type O don't have any of these, so they can only receive other type O blood. Type AB has both (in what's called the ABO group) and can receive any blood. Types A and B have one or the other. There's also the Rh group, which is where the + and - come in. If you're + you can receive both + and -, but if you're - you can only receive other -.

There are actually 33 blood group systems that have been identified, but the ABO and Rh are the two most important. The others don't result in too many severe complications.