That's a great question! Different blood types are named because they have certain traits that characterize the red blood cells. People have a blood type that is determined by their parents (through genetics; similar to some people having brown eyes, while others have blue eyes. AB blood is named that because it has the traits of both A-type red blood cells and B-type red blood cells. There is also O blood type, means they have neither A or B type characteristics. Blood types were first identified in the early 1900's.