What characteristic defines a person with Rh positive blood?

A person with Rh positive blood is defined by the presence of the Rh antigen on the surface of their red blood cells. This antigen, also known as the D antigen, is a specific protein that determines whether an individual's blood type is classified as Rh positive or Rh negative. If the Rh antigen is present, the blood is designated as Rh positive, indicating that the individual has this specific trait.

Understanding the importance of the Rh factor is crucial for blood transfusions, pregnancy, and other medical situations where blood types must match to prevent adverse reactions. Those with Rh negative blood lack this antigen, which can lead to complications if they receive Rh positive blood or during pregnancy if an Rh negative mother has an Rh positive baby. Therefore, the defining characteristic of Rh positive blood is clearly the presence of the Rh antigen.