What is the Rh factor in blood and why does it matter?

The answer

The Rh factor refers to the RhD protein present on the surface of red blood cells in approximately 85% of the population (Rh-positive), while the remaining 15% lack it (Rh-negative). The critical medical significance arises during pregnancy: if an Rh-negative mother carries an Rh-positive fetus, fetal blood cells entering the mother's circulation (usually during delivery) can trigger her immune system to produce anti-RhD antibodies. In subsequent Rh-positive pregnancies, these antibodies cross the placenta and attack the fetus's red blood cells, causing hemolytic disease of the newborn (erythroblastosis fetalis), which can be fatal. The development of RhoGAM (Rh immunoglobulin) in the 1960s, administered to Rh-negative mothers during and after pregnancy, prevents antibody formation and has virtually eliminated this once-common cause of infant mortality.