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While most people think that there are only A, B, and O (positive or negative) blood types, the actually number of blood types (so far discovered) is 32.

Via Popular Science:

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.

It should be noted that the traditional number of blood types (assumed to be 8, accounting for positive and negative variations on A, B, and O) account for the blood types of the majority of people.