In the ABO system, (and Rhesus D system) all blood belongs to one of four major groups: A+/−, B+/−, AB+/−, or O+/−. The presence (+) or absence (−) of the RhD (Rhesus D) antigen is indicated by the plus or minus following the ABO type. But there are more than two hundred minor blood groups that can complicate blood transfusions. These are known as rare blood types.