system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes blood a different "type"?

What's the difference between blood in people that means we have different "types" and why are some incompatible?
gpt
Our blood contains antibodies and antigens. Different people have different antibodies/antigens. Antibodies attack antigens of the same type. Blood type O has no antigens and both A and B types of antibodies. Blood type A has antigen A and antibody B. Blood type B has antigen B and antibody A. Blood type AB has both antigens and neither antibodies. There is a final important antigen, known as D, which people are either positive for (they have it) or negative for (they don't have it). This means that someone whose blood type is A+ has antigens A and D, as well as antibody B.