Your blood type is positive or negative based on whether a specific protein called the Rh factor is present on the surface of your red blood cells. If you have the Rh protein, you are Rh-positive. If you do not, you are Rh-negative. This is separate from your ABO type (A, B, AB, or O), and the two systems combine to create your full blood type, such as A-positive or O-negative.
What Makes A Blood Type Positive Or Negative?
The Rh factor is a protein found on the surface of red blood cells. It is also called the Rh antigen or D antigen. When your blood carries this protein, your blood type is positive. When it does not, your blood type is negative.
You inherit your Rh status from your parents through genetics. Each person has two copies of the Rh gene — one from each parent. If you inherit at least one copy that codes for the Rh protein, you will be Rh-positive. If you inherit two copies that do not code for the protein, you will be Rh-negative.
This means an Rh-positive person can have two Rh-positive genes, or one Rh-positive gene and one Rh-negative gene. An Rh-negative person must have two Rh-negative genes.
The Rh protein itself serves no known essential function in the body. People who lack it are perfectly healthy. The protein only matters in specific medical situations, such as pregnancy and blood transfusions.
How Common Is Rh-Negative Blood?
Rh-negative blood is far less common than Rh-positive blood worldwide. About 85% of people are Rh-positive, and roughly 15% are Rh-negative. These figures vary by region and population.
The distribution is not uniform across ethnic groups. Rh-negative status is most common in people of European descent, where approximately 15% to 17% of the population is Rh-negative. In people of African descent, the rate is lower — around 5% to 8%. In Asian populations, Rh-negative status is rare, affecting roughly 1% or fewer.
Because of this, the overall global percentage of Rh-negative people is lower than the European rate. The commonly cited 15% figure mainly reflects populations of European ancestry.
When you combine ABO type with Rh status, the most common blood type in the United States is O-positive, followed by A-positive. The rarest is AB-negative.
What Is the Rh Blood Group System?
The Rh blood group system is one of more than 40 known human blood group systems. It is the most complex and the second most important after ABO. The system involves dozens of antigens, but the D antigen is the one that determines positive or negative status.
The Rh system was discovered in 1940. Before that, doctors knew that some blood transfusions caused severe reactions even when ABO types matched. The discovery of the Rh factor explained many of those cases.
The name “Rh” comes from the rhesus monkey, which was used in early research on the antigen. Despite the name, the Rh factor is not something humans share with monkeys in any meaningful way — it is simply a naming convention from that era of research.
Other antigens in the Rh system include C, c, E, and e. These can matter in certain medical situations, but they do not determine whether your blood type is positive or negative. Only the D antigen does that.
Why Does Rh Status Matter During Pregnancy?
Rh status becomes critically important when an Rh-negative mother carries an Rh-positive baby. This situation is called Rh incompatibility.
During pregnancy, a small amount of the baby’s blood can enter the mother’s bloodstream. If the mother is Rh-negative and the baby is Rh-positive, the mother’s immune system may recognize the Rh protein as foreign and produce antibodies against it.
This usually does not harm a first pregnancy, because the antibodies typically develop late or after delivery. The real risk is in subsequent pregnancies. If the mother becomes sensitized — meaning her immune system has formed antibodies against Rh-positive blood — those antibodies can cross the placenta and attack the red blood cells of a future Rh-positive baby.
This can lead to a condition called hemolytic disease of the newborn. It can cause anemia, jaundice, and in severe cases, serious complications or death.
The good news is that this condition is largely preventable. A treatment called Rh immune globulin (often known by the brand name RhoGAM) is given to Rh-negative mothers during pregnancy and after delivery. It prevents the mother’s immune system from making antibodies against Rh-positive blood.
Because of this preventive treatment, hemolytic disease of the newborn is now rare in countries where the treatment is routinely available. Rh-negative women who are pregnant or planning to become pregnant should discuss Rh testing and immune globulin with their healthcare provider.
Why Does Rh Status Matter for Blood Transfusions?
Rh status must be matched for blood transfusions. If an Rh-negative person receives Rh-positive blood, their immune system may produce antibodies against the Rh protein. This is called sensitization.
The first exposure may not cause an immediate reaction. But if that person later receives Rh-positive blood again, the antibodies can trigger a serious reaction that destroys red blood cells. This is why Rh-negative patients are generally given Rh-negative blood whenever possible.
There is one exception. In emergencies when Rh-negative blood is not available, some clinicians may give Rh-positive blood to an Rh-negative male or to an Rh-negative woman past childbearing age. This is a clinical judgment based on the risks of delaying transfusion versus the risk of sensitization.
Rh-positive patients can safely receive Rh-positive blood. They can also receive Rh-negative blood, because Rh-negative blood lacks the Rh protein and will not trigger an immune response in someone who is already Rh-positive.
How Is Your Blood Type Determined?
Blood typing is a standard laboratory test. It involves mixing a sample of your blood with antibodies that target specific antigens. If your red blood cells clump together in response to the anti-D antibody, you are Rh-positive. If they do not, you are Rh-negative.
The same test determines your ABO type by testing your blood against anti-A and anti-B antibodies. Results are usually available within hours.
Blood typing is routine in several situations:
- Before any blood transfusion
- During pregnancy, usually at the first prenatal visit
- Before surgery where blood loss is possible
- When donating blood
- For newborn screening in some cases
You can also find out your blood type through a home test kit or by donating blood. Home kits vary in accuracy, so a laboratory test is considered more reliable.
Can Your Blood Type Change?
Your blood type does not change. It is determined by genetics and stays the same throughout your life.
There are rare exceptions that can make it appear as though your blood type has changed. Certain diseases, such as leukemia, can alter the antigens on red blood cells and affect test results. Some infections can also cause misleading results.
Bone marrow transplants are another exception. If you receive bone marrow from a donor with a different blood type, your blood type may gradually shift to match the donor’s. This is because your body begins producing blood cells from the donor’s stem cells.
These situations are uncommon and usually temporary or related to specific medical treatments. For most people, blood type is fixed for life.
What About the Terms “Positive” and “Negative” in Other Contexts?
In blood banking, “positive” and “negative” refer only to the presence or absence of the Rh D antigen. They have nothing to do with whether your blood is healthy or unhealthy, or whether you have a disease.
This can be confusing because the same words are used differently in medical testing. For example, a “positive” test result for an infection usually means the infection is present. A “negative” result usually means it is not. But in blood typing, positive and negative simply describe whether a specific protein is on your red blood cells.
People sometimes assume that Rh-negative blood is better or worse than Rh-positive blood. Neither is true. Both are normal. Rh-negative blood is simply less common in most populations.
Frequently Asked Questions
What does it mean if your blood type is positive?
If your blood type is positive, it means the Rh D antigen is present on your red blood cells. This is the case for about 85% of people worldwide.
Can two Rh-positive parents have an Rh-negative child?
Yes, this is possible if both parents carry one Rh-positive gene and one Rh-negative gene. Each parent can pass on the Rh-negative gene, resulting in an Rh-negative child.
Is Rh-negative blood rare?
Rh-negative blood is uncommon globally, affecting roughly 15% of people overall, but the rate varies widely by population. It is most common in people of European descent and much rarer in Asian populations.
Does being Rh-negative affect your health?
No, being Rh-negative does not affect your general health. It only matters in specific situations such as pregnancy and blood transfusions.
