The Rh factor is a protein found on the surface of red blood cells. If your red blood cells carry this protein, your blood type is Rh-positive (noted with a “+” after your ABO type, like A+ or O+). If they don’t, you’re Rh-negative (A-, O-, etc.). About 85% of people are Rh-positive, making Rh-negative blood relatively uncommon. The Rh factor rarely matters in everyday life, but it becomes critically important during blood transfusions and pregnancy.
What the Rh Protein Actually Is
The Rh factor refers specifically to a protein called the D antigen, one of about 50 antigens in the broader Rh blood group system. This protein sits embedded in the outer membrane of your red blood cells, anchored by a fat-loving structure that threads through the cell wall. It needs a helper protein called RhAG to assemble correctly in the membrane. When doctors or lab tests refer to someone being “Rh-positive” or “Rh-negative,” they’re talking almost exclusively about whether this D antigen is present.
The name comes from the rhesus monkey. In 1940, Karl Landsteiner (the same scientist who discovered ABO blood types) and Alexander Wiener identified the factor by injecting rhesus monkey blood into rabbits and studying the immune response. The antibodies those rabbits produced reacted with a protein on human red blood cells, revealing a blood group system no one had formally described before.
How Rh Factor Is Inherited
You inherit your Rh status from your parents through a single gene. The Rh-positive trait is dominant, meaning you only need one copy of the gene (from either parent) to have the protein on your red blood cells. Rh-negative is recessive, so you must inherit two copies of the negative version, one from each parent, to lack the protein entirely.
This is why two Rh-positive parents can have an Rh-negative child. If both carry one positive and one negative copy, there’s roughly a 25% chance their child inherits the negative version from both sides. Two Rh-negative parents, on the other hand, will always have Rh-negative children, since neither parent has a positive copy to pass along.
How Common Is Rh-Negative Blood?
Rh-negative blood varies dramatically by population. In Britain, about 17% of the population is Rh-negative. In Northern India, that figure drops to around 4.3%. Among people of African and East Asian descent, Rh-negative blood is even rarer. The Basque population of Spain and France has one of the highest known rates of Rh-negative blood in the world, though researchers still debate why.
At the extreme end of rarity is a condition sometimes called “golden blood,” or Rh-null. People with this blood type lack all Rh antigens, not just the D antigen. Fewer than 50 people worldwide have ever been identified with it. Rh-null blood can theoretically be donated to anyone with rare Rh antibody profiles, making it even more universally compatible than O-negative. But people who have it face a serious problem: they can only safely receive Rh-null blood in return, which is nearly impossible to find. Most manage by banking their own blood before planned surgeries and carefully avoiding situations that might require a transfusion.
Why Rh Factor Matters in Transfusions
The core rule is straightforward: Rh-negative patients should receive Rh-negative blood, while Rh-positive patients can safely receive either. If an Rh-negative person receives Rh-positive blood, their immune system may recognize the D antigen as foreign and build antibodies against it. This sensitization might not cause problems during the first exposure, but any future transfusion with Rh-positive blood could trigger a serious reaction, including fever, chills, jaundice, back pain, kidney failure, and in severe cases, death.
This is why O-negative blood is considered the universal donor type for emergencies. It lacks both ABO antigens and the Rh D antigen, so it’s unlikely to provoke an immune response in any recipient. Hospitals keep O-negative blood on hand for trauma situations where there’s no time to test a patient’s blood type.
Rh Incompatibility in Pregnancy
The most well-known complication of Rh factor involves pregnancy. When an Rh-negative mother carries an Rh-positive baby (the baby having inherited the positive gene from the father), small amounts of the baby’s blood can cross into the mother’s bloodstream, usually during delivery, miscarriage, or certain procedures. The mother’s immune system detects the unfamiliar D antigen and begins producing antibodies against it.
The first pregnancy is usually fine. The initial antibodies the mother produces are a type that can’t cross the placenta, so they don’t reach the baby. The danger comes with the next Rh-positive pregnancy. This time, the mother’s immune system remembers the D antigen and rapidly produces a different class of antibody that does cross the placenta. These antibodies attack and destroy the baby’s red blood cells, a condition called hemolytic disease of the fetus and newborn. Depending on severity, this can cause anemia, jaundice, brain damage, heart failure, or stillbirth.
How Rh Incompatibility Is Prevented
Fortunately, this scenario is almost entirely preventable. Rh-negative mothers receive an injection of Rh immune globulin (commonly known by the brand name RhoGAM) during and after pregnancy. This injection contains antibodies against the D antigen that neutralize any fetal blood cells in the mother’s system before her own immune system can react to them. It essentially tricks the body into thinking there’s nothing foreign to respond to.
The standard schedule involves one injection around 26 to 28 weeks of pregnancy and a second within 72 hours after delivery, if the baby turns out to be Rh-positive. Additional doses may be given after a miscarriage, an ectopic pregnancy, amniocentesis, or any event that could cause fetal blood to mix with the mother’s. If the injection is given earlier in pregnancy, repeat doses every 12 weeks help maintain protection. This approach has dramatically reduced the incidence of hemolytic disease since it was introduced in the 1960s.
How Your Rh Factor Is Tested
Rh typing is done alongside ABO typing with a simple blood draw. A small sample of your blood is mixed with a solution containing anti-D antibodies. If your red blood cells clump together, you have the D antigen and you’re Rh-positive. If they don’t, you’re Rh-negative. The result is reported as part of your full blood type: A+, B-, AB+, O-, and so on.
Most people learn their Rh status through routine blood work, a blood donation, or prenatal screening. If you’ve ever donated blood or had a baby, your Rh type is almost certainly already on file. It doesn’t change over your lifetime, so one test is all you need.