Final answer:
A person is considered Rh-positive if they have the Rh D antigen on their erythrocytes. About 85% of the US population is Rh-positive. Rh status is crucial for blood transfusions and pregnancy.
Step-by-step explanation:
A person is considered Rh-positive (Rh+) if they have the Rh D antigen present on their erythrocytes. This Rh D antigen is the most clinically significant of the many Rh antigens identified. In the United States, about 85 percent of the population is Rh-positive, meaning they possess the Rh D antigen on their red blood cells. This Rh factor is independent of the ABO blood group system; therefore, anyone can be Rh-positive regardless of their ABO blood type.
It is important to note that this antigen plays a critical role when it comes to pregnancy and blood transfusions. For example, Rh-negative mothers carrying Rh-positive fetuses can develop antibodies against the Rh D antigen, potentially leading to complications such as hemolytic disease of the newborn in subsequent Rh-positive pregnancies if preventive treatment with RhoGAM is not administered.