The Rh Factor Explained
The Rh factor is the + or − after your blood type. It marks whether the Rh D antigen sits on your red blood cells.
Rh D antigen present on red cells. About 85% of people.
Rh D antigen absent. About 15% of people.
What Is the Rh Factor?
The Rh (Rhesus) system is the second-most important blood group system after ABO. Its key marker is the Rh D antigen, a protein on the red blood cell surface. If you have it, you are Rh positive (like A+, O+); if you do not, you are Rh negative (like A-, O-). Combined with the four ABO groups, Rh gives the 8 blood types.
Why Rh Matters in Transfusions
Rh negative people do not normally have anti-Rh antibodies, but they can form them after being exposed to Rh positive blood. Once sensitised, a future transfusion of Rh positive blood could trigger a reaction. For safety, Rh negative patients are given Rh negative blood whenever possible. See the full compatibility chart.
Rh Factor in Pregnancy
If an Rh negative mother carries an Rh positive baby, her immune system may treat the baby's red cells as foreign and make anti-Rh antibodies. This is called Rh incompatibility (or Rh disease), and it can harm the current or a future pregnancy. An injection of RhoGAM (anti-D immunoglobulin) prevents the mother from forming these antibodies and is routine care today.
The Discovery
The Rh system was identified in 1940 by Karl Landsteiner and Alexander Wiener, decades after Landsteiner's Nobel Prize-winning discovery of the ABO groups. It closed a major gap in transfusion safety.
Related Guides
This page is for education only and is not medical advice.