Which Blood Type Is the Universal Acceptor?

Blood type is categorized based on which antigens are on the red blood cells. There are four antigens, or antigen groups: A, B, AB and O. Each of these can further be divided into positive and negative. For example, a person can have either A- blood or A+ blood. If the cells have a specific type of antigen on top of the main four, called an Rh factor, it is positive. If it is missing, the blood is negative.