Final answer:
The Rh D antigen is the most significant antigen in the Rh factor group, which can lead to hemolytic disease of the newborn if an Rh- mother has an Rh+ fetus and does not receive RhoGAM.
Step-by-step explanation:
The most medically significant antigen in the Rh factor group is known as Rh D. Individuals who possess the Rh D antigen on their erythrocytes are designated as Rh positive (Rh+), while those who lack it are Rh negative (Rh-). About 85 percent of Americans are Rh+. When an Rh- pregnant woman carries an Rh+ fetus, she may produce anti-Rh antibodies that can cross the placenta and harm the developing fetus, leading to a condition called hemolytic disease of the newborn (HDN). This immune response can agglutinate and lyse the fetal erythrocytes. To prevent this, Rh- pregnant women are often treated with RhoGAM, a medication that helps to prevent the formation of these harmful antibodies.