Turns out there's much more than just A, B, AB, and O: there are now 28 other, rarer types, often named after the person in whom they were discovered. These rarer types are identified by the presence of a particular group of antigens (substances that tell your immune system to send out antibodies), and many, like the Kell and MNS blood types, can actually be concurrent with more common blood types like A or O.