What is the risk of giving Rh-positive blood to an Rh-negative patient on the first transfusion?

Giving Rh-positive blood to an Rh-negative patient on the first transfusion carries the risk of the development of anti-Rh antibodies. When an Rh-negative person receives Rh-positive blood, their immune system may recognize the Rh antigens on the transfused red blood cells as foreign substances. This can lead to the production of antibodies against the Rh factor, a process known as sensitization.

The first transfusion might not cause immediate hemolytic reactions because the immune response takes time to develop—although the person may remain asymptomatic, they are at risk for future transfusions. If they receive Rh-positive blood again after having developed anti-Rh antibodies, serious complications such as hemolytic transfusion reactions can occur.

Immediate hemolytic reactions are more commonly associated with incompatible blood types and often occur upon the first infusion in cases of AB0 incompatibility rather than Rh incompatibility. Sever allergic reactions are not specifically related to Rh factor incompatibilities, and while there's always a risk of infection with any blood transfusion, this risk is not influenced by the Rh status.