In addition to the ABO blood grouping system, another prominent one is the Rh blood group system.
About two-thirds of the population contains a third antigen on the surface of their red blood cells known as the Rh factor or Rh antigen; this decides whether the blood group is positive or negative.
If the Rh factor is present, an individual is rhesus positive (Rh+ve).
If an Rh factor is absent, the individual is called rhesus negative (Rh-ve) as they generally produce Rh antibodies.
Therefore, compatibility between donor and individual is crucial in this case as well.