system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5- What are blood groups? Why do they differ from person to person? Why can't a person of one blood group receive blood from a person of a different blood group?

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gpt
In some blood types, it's like the blood cells are wearing little hats. If you get some blood cells wearing hats and you don't normally have them, your body is like, whoah, these shouldn't be here, and it attacks them. Type A only likes type A hats, and type B only likes type B hats. Type AB is cool with either kind of hat. Type O doesn't have any hats itself, so the bodies of Type O people will attack A or B or AB blood cells. People with A, B, or AB blood types are ok receiving the hatless Type O blood cells.

The positive and negative refers to Rh blood types. Rh blood types are like the cells have mustaches. Rh+ types, they've all got mustaches but they're ok accepting blood cells with no mustaches. Rh- types, they don't want any of those stupid mustaches. Maybe they could tolerate it a bit, if it's an emergency, but really no mustaches would be better. So an AB+ person is cool with blood that has any kind of hat and mustaches or no mustaches, so they can take any kind of blood, while an O- person can only accept blood with no hats and no mustaches.

There are other accessories that affect blood type, representing more detailed blood types. For surgery and stuff usually ABO type and Rh type are all that matters, but for some transplants you really need the closest type possible, like blood with the exact same hat and facial hair and jewelry and clothes and haircut. That's why people who need kidneys or bone marrow will sometimes ask lots and lots of people to check if they're a potential match, and maybe never find anyone.