There are eight main blood types, determined by two factors: the ABO group (A, B, AB, or O) and the Rh factor (positive or negative). That gives us A+, A-, B+, B-, AB+, AB-, O+, and O-. Your blood type is set at birth, inherited from your parents, and never changes. It matters most during blood transfusions and pregnancy, where a mismatch can cause serious complications.
The ABO System
The ABO system sorts blood into four groups based on molecules called antigens sitting on the surface of your red blood cells. Type A blood carries the A antigen. Type B carries the B antigen. Type AB carries both. Type O carries neither.
Your immune system treats unfamiliar antigens as threats. If you have type A blood, your body naturally produces antibodies against the B antigen, and vice versa. Type O blood carries antibodies against both A and B. Type AB carries no antibodies against either. This is why mixing the wrong blood types during a transfusion can trigger a dangerous immune reaction: your antibodies attack the foreign red blood cells.
The Rh Factor
The second part of your blood type is the Rh factor, a protein on the surface of red blood cells. If you have it, you’re Rh-positive (the “+” in O+ or A+). If you don’t, you’re Rh-negative. About 85% of people are Rh-positive.
The Rh factor is especially important during pregnancy. If an Rh-negative mother carries an Rh-positive baby, her immune system can start producing antibodies against the baby’s blood cells. Before preventive treatment became available in the 1970s, this happened in roughly 13 to 16% of at-risk pregnancies. Today, with a targeted injection given during and after pregnancy, that rate has dropped to about 0.1 to 0.2%.
How Blood Type Is Inherited
You inherit one copy of the blood type gene from each parent, and the combination of those two copies determines your type. There are three versions (alleles) of the gene: A, B, and O. The A allele produces the A antigen, the B allele produces the B antigen, and the O allele produces nothing functional.
A and B are both dominant over O, and they’re co-dominant with each other. That means if you inherit an A from one parent and a B from the other, you’ll be type AB. If you inherit an A and an O, you’ll be type A. Two O alleles give you type O. This is why two parents with type A blood can have a type O child: both parents can silently carry one O allele and pass it along.
The Rh factor follows a similar pattern. Rh-positive is dominant, so you only need one copy to be positive. Two Rh-negative parents will always have Rh-negative children.
How Common Each Blood Type Is
Blood type distribution varies by population, but in the United States the breakdown looks like this:
- O+: 37.4% (the most common)
- A+: 35.7%
- B+: 8.5%
- AB+: 3.4%
- O-: 6.6%
- A-: 6.3%
- B-: 1.5%
- AB-: 0.6% (the rarest)
Together, O+ and A+ account for nearly three-quarters of the population. The negative blood types are far less common across the board, which is why blood banks are often short on Rh-negative supply.
Who Can Donate to Whom
Blood transfusion compatibility follows a straightforward logic based on which antigens the recipient’s immune system will tolerate.
For red blood cell transfusions, type O negative is the universal donor. Because O-negative red blood cells carry no A, B, or Rh antigens, virtually anyone can receive them without an immune reaction. This makes O-negative blood critical in emergencies when there’s no time to test a patient’s type. On the other end, type AB positive is the universal recipient for red blood cells, since AB+ individuals have all three antigens and won’t reject any of them.
The general rules for red cell donation:
- Type O can donate red blood cells to anyone.
- Type A can donate to A and AB.
- Type B can donate to B and AB.
- Type AB can donate red cells only to other AB individuals.
Rh-negative blood can go to both Rh-positive and Rh-negative recipients, but Rh-positive blood should only go to Rh-positive recipients.
For plasma transfusions, the rules flip. Plasma compatibility depends on antibodies rather than antigens, so type AB is the universal plasma donor (AB plasma contains no anti-A or anti-B antibodies), while type O recipients can only receive O plasma.
Beyond ABO and Rh
ABO and Rh are the two blood group systems that matter most in everyday medicine, but they’re far from the only ones. The International Society of Blood Transfusion currently recognizes 48 distinct blood group systems, each defined by different proteins or sugars on the surface of red blood cells. Most of these rarely cause problems in transfusions, but they can become relevant for people who receive frequent transfusions, such as those with sickle cell disease, or for patients who develop unusual antibodies after previous transfusions or pregnancies.
How Your Blood Type Is Tested
Blood typing is a simple lab test that takes minutes. A technician mixes a small sample of your blood with solutions containing antibodies against A, B, and Rh antigens. If your red blood cells clump together when exposed to anti-A antibodies, you have the A antigen. If they clump with anti-B, you have B. If they clump with both, you’re AB. If neither causes clumping, you’re type O. A separate test with anti-Rh antibodies determines your positive or negative status.
Before any transfusion, hospitals also perform a crossmatch, mixing a small amount of the donor blood with the recipient’s blood to confirm compatibility. Your blood type is typically recorded in your medical chart after your first test, but hospitals will verify it again before every transfusion rather than relying on records alone.