ABO and Rhesus are the two commonly used method of typing the blood. Under the ABO system, the blood can be A, B, AB or O. The percentage of the different ABO groups in the population is approximately A(23%), B(23%), AB(5%) and O(49%). Rhesus grouping is reported as positive or negative. Rhesus grouping becomes important when a Rhesus Negative woman becomes pregnant with a Rhesus Positive baby. But with modern medical treatment, this is not a problem provided the diagnosis is made early. Blood grouping is important when a person requires a blood transfusion. There is no such thing as a bad blood group.