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There are 4 different blood types: A, B, O, and AB. How does your body know which blood type you are? There are antigens – markers – on the outside of every cell. There are antigens that say, "I'm a red blood cell", "I'm type A blood", and even antigens that say, "I'm me." Bacteria have antigens that say, "I'm a bacteria." So any given cell could have lots of different antigens.
