What is the difference between ab negative and ab positive blood group?

Blood typing: When one says that they have a-, b-, o-, or ab- blood they mean to say that they are rhesus factor negative. The rh blood group system is second only to the ABO system in importance in transfusion medicine. Antibodies to rh antigens such as d, c, and e arise following transfusions and pregnancy. They are responsible for hemolytic disease of the newborn and severe hemolytic transfusion reactions.