There are several different ways of 'grouping' blood. One system has blood groups A, B, AB and O. Another system is the rhesus grouping where a blood is classified as rhesus-positive or -negative; D and Rh are the same group. It is a combination of these 2 classificatons which is used most commonly. These are the most commonly known groups, but there are 28 additional systems that are recognised around the world.