A “positive” blood type means your red blood cells carry a specific protein called the Rh factor on their surface. About 85% of people in the United States are Rh positive, making it far more common than Rh negative. The positive or negative label is the “+” or “−” you see after your letter blood type, so “O+” means type O blood that is Rh positive.
What the Rh Factor Actually Is
Your blood type has two parts. The first is the ABO group (A, B, AB, or O), which describes one set of markers on your red blood cells. The second is the Rh factor, an inherited protein either present or absent on those same cells. If the protein is there, you’re positive. If it’s not, you’re negative.
This protein matters because your immune system is trained to attack unfamiliar substances. Someone who is Rh negative will produce antibodies against Rh-positive blood if exposed to it through a transfusion or pregnancy. Someone who is Rh positive, on the other hand, won’t react to either Rh-positive or Rh-negative blood, since the protein is already familiar to their body.
How Common Each Positive Blood Type Is
In the United States, the four Rh-positive blood types account for roughly 85% of the population:
- O positive: 37.4%, the most common blood type overall
- A positive: 35.7%, nearly as common
- B positive: 8.5%
- AB positive: 3.4%, the rarest of the positive types
Globally, the pattern shifts slightly. O positive remains the most common at about 38.7%, but B positive is more prevalent worldwide (22%) than it is in the U.S., reflecting higher rates in parts of Asia and Africa. A positive sits at 27.4% globally, and AB positive at 5.9%.
How You Inherit a Positive Blood Type
You receive one Rh gene from each parent. The Rh-positive gene is dominant, meaning it overrides the negative version whenever the two are paired together. If you carry two positive copies (+ +) or one positive and one negative (+ −), your blood type will test as positive. The only way to be Rh negative is to inherit two negative copies (− −), one from each parent.
This is why two Rh-positive parents can still have an Rh-negative child. If both parents carry a hidden negative gene (+ −), there’s a one-in-four chance their child inherits − − and ends up Rh negative. If either parent is + +, every child will be Rh positive regardless of the other parent’s status.
Which Blood Types You Can Receive
Being Rh positive gives you more flexibility when receiving blood. You can safely accept both Rh-positive and Rh-negative red blood cells, as long as the ABO group also matches. Here’s what each positive type can receive:
- O positive: O+, O−
- A positive: A+, A−, O+, O−
- B positive: B+, B−, O+, O−
- AB positive: any blood type (this is why AB+ is called the universal recipient)
AB positive earns the “universal recipient” label because these individuals have all major surface markers already present on their red blood cells. Their immune system recognizes everything as familiar, so no transfusion reaction occurs regardless of donor type.
What Happens in Emergencies
When someone needs blood urgently and there’s no time to test their type, hospitals default to O-positive red blood cells for most adult men and postmenopausal women. O positive works because type O cells lack the A and B markers that would trigger a reaction in other blood types, and Rh-positive blood is safe for the vast majority of the population.
The exception is women of childbearing age. They receive O-negative blood in emergencies to avoid the small risk of Rh sensitization, which could cause complications in a future pregnancy. Once testing is complete, patients are switched to their exact type-specific blood.
Why Rh Factor Matters in Pregnancy
Rh status becomes especially important when an Rh-negative woman carries an Rh-positive baby. During pregnancy, small amounts of fetal blood can cross into the mother’s bloodstream, particularly during labor, bleeding episodes, or certain prenatal procedures. When this happens, the mother’s immune system recognizes the Rh protein as foreign and begins producing antibodies against it.
This process, called Rh sensitization, rarely causes problems in a first pregnancy because the antibody buildup is slow. The danger comes with subsequent pregnancies. If the mother carries another Rh-positive baby, those antibodies can cross the placenta and attack the baby’s red blood cells, potentially causing severe anemia or worse. Doctors prevent this by giving Rh-negative mothers a preventive injection during pregnancy that stops the immune response before it starts.
If you’re Rh positive, this particular complication doesn’t apply to you. Your body already has the Rh protein, so your immune system won’t react to an Rh-positive baby’s blood.
Does Positive Blood Affect Your Health?
Your Rh status has no meaningful impact on day-to-day health. One area researchers have looked at is infectious disease susceptibility. A hospital-based study of over 2,500 COVID-19 patients in Delhi found that Rh-positive individuals appeared slightly more susceptible to infection compared to Rh-negative individuals. However, Rh status showed no association with disease severity or mortality, and Rh-positive patients actually had shorter hospital stays on average. Given that over 92% of the general population in that study was already Rh positive, the practical significance of this difference is limited.
Outside of transfusions and pregnancy, being Rh positive or negative doesn’t change what you should eat, how you exercise, or what diseases you need to screen for. The main reason to know your Rh status is so that medical teams can match your blood correctly if you ever need a transfusion, and so that prenatal care can be tailored appropriately if you become pregnant.