The eight blood types are A+, A-, B+, B-, AB+, AB-, O+, and O-. They’re determined by two things: which antigens (markers) sit on the surface of your red blood cells, and whether a protein called the Rh factor is present. Understanding these types matters most in transfusions and pregnancy, where a mismatch can trigger a dangerous immune reaction.
How the 8 Types Are Determined
Your blood type comes from two separate classification systems layered on top of each other. The first is the ABO system, which sorts everyone into four groups based on antigens, small marker molecules on the outside of red blood cells. If your cells carry the A antigen, you’re group A. If they carry the B antigen, you’re group B. If they carry both, you’re AB. If they carry neither, you’re group O.
The second system is the Rh system, which checks for a specific protein called the RhD antigen. If your red blood cells have it, you’re positive (+). If they don’t, you’re negative (-). Combine ABO with Rh and you get eight possible types: A+, A-, B+, B-, AB+, AB-, O+, and O-.
These eight are the most clinically important, but they’re not the whole picture. Scientists have identified 48 blood group systems in total, encompassing hundreds of antigens. For everyday medicine, though, ABO and Rh are what hospitals test for first.
All 8 Blood Types at a Glance
Not all blood types are equally common. Based on donor data from NHS Blood and Transplant (as of early 2026), here’s how the population breaks down:
- O+: 36%, the most common type overall
- A+: 28%
- B+: 8%
- AB+: 2%
- O-: 14%
- A-: 8%
- B-: 3%
- AB-: 1%, the rarest of the eight
These numbers shift somewhat between ethnic groups and regions, but the overall pattern holds: O and A types dominate, while AB types are uncommon. Negative types are always less prevalent than their positive counterparts because having the Rh factor is the more common trait.
What Makes Each Type Compatible for Transfusion
Your immune system produces antibodies against whichever ABO antigens your own red blood cells lack. If you’re type A, your blood naturally contains antibodies that attack B antigens. If you’re type B, you carry antibodies against A. Type O carries antibodies against both A and B, while type AB carries neither. This is why getting the wrong blood type during a transfusion can cause a severe, potentially fatal reaction: your antibodies attack the donated red blood cells as though they’re foreign invaders.
The Rh factor adds another layer. Rh-negative blood can generally be given to Rh-positive patients, but Rh-positive blood should not go to Rh-negative patients because it can trigger the production of anti-Rh antibodies.
Here’s how the donation rules work for red blood cells:
- Type O- can donate red blood cells to anyone, making it the universal red cell donor. This is the type hospitals reach for in emergencies when there’s no time to test.
- Type O+ can donate to any positive type (A+, B+, AB+, O+).
- Type A can donate to A and AB recipients.
- Type B can donate to B and AB recipients.
- Type AB+ is the universal recipient for red blood cells, able to receive from all eight types because it carries every antigen and produces no ABO antibodies.
- Type AB- can receive from any negative type.
Plasma Works in Reverse
Plasma compatibility flips the red blood cell rules. Since plasma contains the antibodies (not the antigens), the concern is whether the donor’s antibodies will attack the recipient’s cells. Type AB plasma has no anti-A or anti-B antibodies, so it’s safe for nearly everyone, making AB the universal plasma donor. Type O plasma, on the other hand, contains antibodies against both A and B, so it’s most restricted in who can receive it.
One practical difference: the Rh factor doesn’t matter much for plasma transfusions. Plasma of any Rh type can generally be given regardless of the recipient’s Rh status.
How You Inherit Your Blood Type
Your ABO type is determined by a single gene with three possible versions (alleles): one that codes for the A antigen, one for B, and one that codes for neither (O). You inherit one allele from each parent, giving you two copies total.
A and B are codominant, meaning if you get one of each, both antigens show up on your cells and you’re type AB. The O allele is recessive, so it only determines your blood type if you inherit it from both parents. That means a person who’s type A could carry either two A alleles or one A and one O. Two parents who are both type A can have a type O child if they each pass along their hidden O allele.
The Rh factor follows a similar pattern. The gene for having the RhD protein is dominant over the gene for lacking it. So you’ll be Rh-positive if you inherit even one copy of the positive version. You’re only Rh-negative if both parents contribute the negative version. This is why Rh-negative types are less common across all ABO groups.
Why Rh Factor Matters in Pregnancy
If an Rh-negative mother carries an Rh-positive baby (which happens when the father is Rh-positive), her immune system may recognize the baby’s RhD protein as foreign and produce antibodies against it. This typically isn’t a problem in a first pregnancy, but in later pregnancies those antibodies can cross the placenta and attack the baby’s red blood cells, causing a condition called hemolytic disease of the newborn. Routine screening and preventive treatment during pregnancy have made serious complications rare, but this is one of the main reasons blood type is checked early in prenatal care.
How to Find Out Your Blood Type
Your blood type is tested if you donate blood, have surgery, or receive prenatal care. It’s also recorded if you’ve ever been hospitalized for a procedure involving transfusion. If you’ve never been tested, a simple blood draw at your doctor’s office will identify your ABO group and Rh status. At-home testing kits that use a finger prick are also available, though lab testing is more reliable for medical purposes.
Once determined, your blood type never changes. It’s fixed at conception by the genes you inherited, and it stays the same for life.