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Why is Rh-positive blood usually avoided for Rh-negative recipients?

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Blood plasma — blood Blood sediment in test tubes, showing plasma (clear fluid), red blood cells (red), and the release of hemoglobin into the surrounding plasma (pink).

Rh-positive blood is typically avoided for Rh-negative recipients because the Rh-negative individual lacks the Rh antigen (specifically the RhD antigen) on their red blood cells.12

When an Rh-negative person receives Rh-positive blood, their immune system may recognize the foreign antigen and produce anti-Rh antibodies. While an initial transfusion might not trigger an immediate reaction, the development of these antibodies sensitizes the recipient. If the individual is exposed to Rh-positive blood again, these antibodies will attack the foreign red blood cells, causing them to clump together (agglutination) and undergo hemolysis, which can lead to severe illness or death.12

Furthermore, for females of childbearing age, this sensitization poses a significant risk for future pregnancies. If a sensitized woman carries an Rh-positive fetus, her anti-Rh antibodies can cross the placenta and attack the fetus's blood cells, potentially causing erythroblastosis fetalis, a condition that can be fatal to the infant. Consequently, medical professionals prioritize using Rh-negative blood for these recipients to prevent both transfusion reactions and future reproductive complications.213