Why are Rh-negative patients usually given Rh-negative red blood cells?

Rh-negative patients are typically given Rh-negative red blood cells to prevent the development of anti-Rh antibodies, which occur in 50 to 75 percent of Rh-negative individuals exposed to Rh-positive blood.1

When an Rh-negative person receives Rh-positive blood, their immune system recognizes the foreign Rh antigen and produces anti-Rh antibodies. While the initial transfusion may not always cause an immediate reaction, these antibodies will attack foreign red blood cells during any subsequent exposure, leading to hemolysis—the destruction of red blood cells—which can cause serious illness or death.12

Furthermore, this precaution is critical for females of childbearing age. If an Rh-negative woman develops anti-Rh antibodies, those antibodies can cross the placenta during a future pregnancy and attack an Rh-positive fetus. This can result in erythroblastosis fetalis, a potentially fatal condition for the newborn. Consequently, Rh-positive blood should only be administered to Rh-negative females in life-threatening, emergency situations where no other compatible blood is available.12