The “positive” or “negative” in your blood type refers to a single protein on the surface of your red blood cells called the RhD protein. If your red blood cells carry this protein, you’re Rh positive. If they don’t, you’re Rh negative. About 85% of people in the United States are Rh positive, while the remaining 15% are Rh negative.
The RhD Protein
Your red blood cells are covered in hundreds of different surface proteins, and the immune system uses these proteins to tell the difference between “self” and “foreign.” The RhD protein is one of the most medically important. It’s a large molecule, over 400 amino acids long, that threads back and forth through the red blood cell membrane 12 times, anchoring itself deeply into the cell’s structure.
The name “Rh” comes from the rhesus monkey. In 1937, scientists Karl Landsteiner and Alexander Weiner discovered this blood group system while studying rhesus macaque blood. The protein they identified turned out to be shared across primates, including humans.
How You Inherit Your Rh Status
Whether you’re positive or negative comes down to the RHD gene you inherit from each parent. Having the gene is dominant, meaning you only need one working copy to produce the RhD protein and test positive. To be Rh negative, you need to inherit two copies of the deletion, one from each parent.
This is why two Rh-positive parents can have an Rh-negative child. If both parents carry one working copy and one deleted copy, there’s a 25% chance their child inherits the deletion from both sides and ends up Rh negative. The most common cause of Rh negativity, especially among Europeans, is a complete deletion of the RHD gene. During cell division, two highly similar DNA segments misalign and recombine unevenly, erasing the gene entirely.
About 41% of the population carries at least one copy of this deletion. Roughly 17% are homozygous for it, meaning they inherited the deletion from both parents and are Rh negative.
How Rh Status Varies by Population
Rh-negative blood is not evenly distributed around the world. It’s most common among people of European descent: about 17% of the British population and 15% of the US population is Rh negative. The Basque people of Spain and France, along with certain populations in the High Atlas mountains of Morocco, have the highest known rates, around 29%. One study in Saudi Arabia found a similarly high rate.
In most of Asia, Rh negativity is rare. Less than 1% of people in China, Japan, and Indonesia are Rh negative. In India, rates range from about 0.6% to 8.4% depending on the region. Across sub-Saharan Africa, Rh negativity typically falls between 1% and 3%, with a few exceptions like the Yoruba people of Nigeria, where the rate is closer to 6%.
How It’s Tested
Blood typing is straightforward. In the most common method, a lab technician mixes a small sample of your red blood cells with a reagent containing anti-D antibodies. If the RhD protein is present on your cells, the antibodies latch on and cause the cells to clump together visibly. This clumping, called agglutination, means you’re Rh positive. No clumping means Rh negative.
This test can be done using test tubes, gel columns, or solid-phase plates. More recently, DNA-based methods like PCR and gene sequencing can identify your Rh status at the genetic level, which is especially useful in complex cases or for prenatal testing when a blood sample from the fetus isn’t available.
Why It Matters for Transfusions
If you’re Rh negative and receive Rh-positive blood, your immune system may recognize the RhD protein as foreign and build antibodies against it. This isn’t usually dangerous during a first exposure, but once those antibodies exist, a second exposure to Rh-positive blood can trigger a serious immune reaction that destroys the transfused red blood cells.
The practical rules are simple. Rh-positive patients can safely receive either Rh-positive or Rh-negative blood. Rh-negative patients should receive Rh-negative blood whenever possible, though Rh-positive blood may be given in emergencies when Rh-negative units aren’t available.
Why It Matters During Pregnancy
The most significant medical concern around Rh status involves pregnancy. When an Rh-negative mother carries an Rh-positive baby, small amounts of the baby’s blood can cross the placenta and enter the mother’s bloodstream, particularly during delivery. The mother’s immune system detects the unfamiliar RhD protein and starts producing antibodies against it.
This first pregnancy is usually fine. The problem arises in a subsequent pregnancy with another Rh-positive baby. Those antibodies, now already present in the mother’s blood, can cross the placenta in the other direction and attack the new baby’s red blood cells, causing a condition called hemolytic disease of the newborn. The result is anemia in the fetus, which can range from mild to life-threatening.
To prevent this, Rh-negative mothers receive an injection of Rh immune globulin (commonly known by the brand name RhoGAM) during pregnancy and after delivery. This treatment works by suppressing the mother’s immune response before she can form lasting antibodies. If the father is heterozygous for the RHD gene (carrying one copy), there’s a 50% chance the baby will be Rh negative too, in which case no immune conflict occurs.
Rh-Null: When All Rh Proteins Are Missing
There’s a condition far rarer than being Rh negative. People with Rh-null blood lack not just the RhD protein but all Rh proteins on their red blood cells. This affects roughly 1 in 6 million people worldwide and is inherited in an autosomal recessive pattern, meaning both parents must carry the genetic variant.
Unlike standard Rh-negative blood, which causes no health problems on its own, Rh-null blood comes with consequences. The Rh proteins play a structural role in the red blood cell membrane, so without them, the cells become fragile and misshapen. People with Rh-null blood typically experience chronic hemolytic anemia, a condition where red blood cells break down faster than the body can replace them. The severity varies, but the cells often show visible abnormalities under a microscope, including unusual shapes and increased fragility. Because these individuals can only receive blood from other Rh-null donors, their blood is sometimes called “golden blood” for its extreme rarity and universal compatibility within the Rh system.