The “positive” or “negative” in your blood type refers to whether your red blood cells carry a specific protein called the Rh factor on their surface. If the protein is present, you’re Rh-positive (like A+ or O+). If it’s absent, you’re Rh-negative (like A− or O−). About 85% of people worldwide are Rh-positive, making Rh-negative the less common variant.
What the Rh Factor Actually Is
Your blood type has two parts. The letter (A, B, AB, or O) describes which sugar molecules sit on the surface of your red blood cells. The positive or negative describes something entirely separate: whether your cells also carry a protein called RhD. This protein is embedded in the membrane of each red blood cell, and your body either makes it or it doesn’t. There’s no in-between.
The name “Rh” comes from rhesus monkeys, whose red blood cells were used in the original experiments that identified the protein in humans. Despite the name, the relevance is entirely human. The RhD protein is one of the most reactive substances in the blood system, meaning the immune system responds strongly when it encounters RhD on foreign blood cells and doesn’t recognize it as its own.
How You Inherit Positive or Negative
You inherit your Rh status from your parents through a single gene called RHD. The version of the gene that produces the RhD protein is dominant, and the version that doesn’t produce it (caused by a deletion in the gene) is recessive. This means you only need one copy of the active gene, from either parent, to be Rh-positive.
To be Rh-negative, you need two copies of the deleted version, one from each parent. Two Rh-positive parents can still have an Rh-negative child if both carry one silent copy of the deletion. A parent who is Rh-positive but carries one deleted copy (heterozygous) has a 50% chance of passing the negative version to each child.
How Common Is Rh-Negative Blood?
Rh-negative blood is relatively rare, and its frequency varies dramatically by ethnicity. About 18% of people of European descent are Rh-negative, compared to roughly 4% of people of African descent, 4% of Latino Americans, and just 1% of people of Asian descent. In large population studies from regions like South Asia, Rh negativity runs around 4.3%, compared to about 17% in Britain.
This uneven distribution matters practically. Blood banks in areas with large European-descended populations tend to have more Rh-negative blood available, while hospitals in other regions may face shortages of Rh-negative units when they’re needed.
Why It Matters for Blood Transfusions
If you’re Rh-negative and receive Rh-positive blood, your immune system can recognize the RhD protein as foreign and build antibodies against it. This process is called alloimmunization. It may not cause problems during a first exposure, but if you receive Rh-positive blood a second time, those antibodies can attack the transfused red blood cells, destroying them. This is called a hemolytic transfusion reaction, and it can range from mild fever and jaundice to a severe, life-threatening emergency depending on how the immune system attacks the cells.
For this reason, Rh-negative patients are given Rh-negative blood whenever possible. Rh-positive patients, on the other hand, can safely receive either Rh-positive or Rh-negative blood because their immune system already recognizes the RhD protein as normal.
This is also why O-negative blood is considered the true universal donor type. O-type red blood cells lack the A and B sugar molecules, so they don’t trigger antibodies against those markers. And being Rh-negative means they won’t trigger an Rh response either. Only about 1 to 8% of the U.S. population is O-negative, which is why it’s constantly in short supply at blood banks. O-positive blood, held by 37 to 53% of the population, works as a universal donor within the much larger group of Rh-positive recipients.
Rh Factor and Pregnancy
The most significant real-world consequence of Rh-negative status involves pregnancy. When an Rh-negative mother carries an Rh-positive baby (inheriting the father’s RhD gene), a small amount of fetal blood can cross into the mother’s bloodstream during pregnancy or delivery. As little as 0.1 mL of fetal blood is enough to trigger the mother’s immune system to start producing anti-D antibodies.
This first pregnancy usually proceeds without major issues because the initial antibodies produced are a type that can’t cross the placenta. The problem arises in subsequent pregnancies. If the mother becomes pregnant again with another Rh-positive baby, her immune system remembers the RhD protein and rapidly produces a different class of antibodies that can cross the placenta. These antibodies enter the fetal bloodstream and begin destroying the baby’s red blood cells, a condition called hemolytic disease of the fetus and newborn. In severe cases, fetal anemia can lead to a dangerous condition involving widespread swelling, fluid buildup around the lungs and heart, and organ failure.
The severity tends to increase with each successive Rh-positive pregnancy because the mother’s immune memory strengthens over time.
How Rh Complications Are Prevented
Rh-negative mothers carrying Rh-positive babies receive an injection of Rh immune globulin (commonly known by the brand name RhoGAM) during pregnancy and again after delivery. This injection works by neutralizing any fetal RhD-positive blood cells that enter the mother’s circulation before her immune system has a chance to recognize them and build its own antibodies. It’s essentially a preemptive strike that prevents sensitization from ever happening.
This preventive treatment has been one of the most successful interventions in modern obstetrics. Before it became routine, hemolytic disease of the newborn was a leading cause of infant illness and death in Rh-incompatible pregnancies. Today, serious complications are rare when the injections are given on schedule.
Does Being Rh-Negative Affect Your Health?
Outside of transfusion and pregnancy, your Rh status has no known effect on your daily health. Being Rh-negative doesn’t make you more susceptible to illness, and being Rh-positive doesn’t offer any particular health advantage. The distinction is purely immunological: it only becomes relevant when Rh-negative blood encounters Rh-positive blood cells from an outside source.
If you don’t know your Rh status, it’s determined through a simple blood test, the same one used to identify your ABO type. Knowing whether you’re positive or negative is especially important if you might need a blood transfusion, are planning a pregnancy, or want to donate blood.