A/B/AB/O are blood types. A- types can only donate to other A people (+ or – doesnt matter to the recipient) B- types can only donate to other B people, AB- can donate to A, B, and AB and O- can donate to anyone. Type A has A antigens (as in antigen types, which are the macromolecules on the surface of the cell) Type B has B antigens, type AB has A and B antigens and Type O has none. It’s a bit more complex but since i cant send links i recommend you google it