system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are there different blood types? What makes them A, B, O?

Why can blood type AB receive from all types but can't donate to any other one?

Why can type O donate universally but can't receive any other blood or else the body will go into shock?
gpt
You're forgetting what's called the "Rh factor", usually denoted by "+" or "-". A, B, and + denote little "flags", if you will, on the outside of red blood cells.  O denotes the absence of an A or B "flag" and "-" denotes the absence of a + "flag".  Your body will attack cells with any "flags" that it finds that are unfamiliar to it. So a person with AB+ blood can accept any type blood because their body doesn't attack A, B, or + "flags" and O and - have no "flags".  But AB+ people can't donate to anyone except AB+, because otherwise the person will lack at least one "flag" and their body will attack the donated blood. And that's why a person with O- blood can donate blood to anyone, but can only receive from other O-: O- doesn't have any "flags", so no one's body will attack those cells, but O- people will attack any and all "flags".


In reality, there are many more "flags" than just A, B, and +, but they're the "big three" that cause most problems with blood transfusions.