Your blood type is determined by proteins called antigens on the surface of your red blood cells, combined with antibodies floating in your plasma. Two systems matter most: the ABO system, which sorts you into type A, B, AB, or O, and the Rh system, which adds a positive or negative label. Together, they produce the eight common blood types you’ve seen on donor cards and lab results.
The ABO System
Every red blood cell can carry A antigens, B antigens, both, or neither. That’s the entire basis of ABO typing. If your cells carry the A antigen, you’re type A. If they carry the B antigen, you’re type B. Carry both, and you’re AB. Carry neither, and you’re type O.
What makes this system critical for transfusions is the other half of the equation: your plasma automatically contains antibodies against whichever antigens you lack. Type A blood carries anti-B antibodies. Type B carries anti-A. Type O carries both anti-A and anti-B, which is why giving type A or B blood to a type O person triggers a potentially dangerous immune reaction. Type AB carries no ABO antibodies at all, which is why AB-positive individuals can receive red blood cells from any blood type.
How You Inherit Your Blood Type
Blood type follows straightforward genetic rules. You inherit one allele from each parent, giving you two total. There are three possible alleles: A, B, and O. The A and B alleles are codominant, meaning if you inherit one of each, both express themselves and you end up with type AB. The O allele is recessive, so it only shows up when you inherit it from both parents.
This is why two parents who are both type A can have a type O child. If each parent carries one A allele and one hidden O allele (genotype AO), there’s a one-in-four chance their child inherits O from both sides. The same applies to type B parents with a BO genotype. Meanwhile, a type AB parent will always pass on either an A or a B allele, never O, so they can never have a type O child regardless of the other parent’s type.
The Rh Factor: Positive vs. Negative
The second major label on your blood type comes from a protein called the Rh D antigen. If your red blood cells carry this protein, you’re Rh-positive. If they don’t, you’re Rh-negative. It’s that simple, and it works independently of the ABO system.
Most people are Rh-positive. Among blood donors in the UK, about 74% are positive and 26% are negative. Rh status matters most during pregnancy: if an Rh-negative mother carries an Rh-positive baby (inheriting the D antigen from the father), her immune system can develop antibodies against the baby’s blood cells. This is preventable with routine screening and treatment, but it’s one reason Rh typing is done early in pregnancy.
How Blood Typing Works in the Lab
Determining your blood type requires two complementary tests, and labs run both to double-check the result.
The first is called forward typing. A technician takes a sample of your red blood cells and mixes them with known antibodies: anti-A, anti-B, and anti-D. If your cells clump together (agglutinate) when mixed with anti-A, you have the A antigen. If they clump with anti-B, you have the B antigen. If they clump with anti-D, you’re Rh-positive. No clumping means the antigen is absent.
The second test, reverse typing, works in the opposite direction. Your plasma is mixed with known type A and type B red blood cells. If your plasma causes the A cells to clump, you carry anti-A antibodies. If it clumps B cells, you carry anti-B. The results of both tests should agree. When they don’t, it signals something unusual that requires further investigation.
Blood Type Distribution
Blood types are not evenly distributed. Based on donor data from the UK (as of 2026), the breakdown looks like this:
- O positive: 36%
- A positive: 28%
- B positive: 8%
- AB positive: 2%
- O negative: 14%
- A negative: 8%
- B negative: 3%
- AB negative: 1%
These numbers shift depending on ethnic background and geography. Type B is more common in South and Central Asian populations, for instance, while type O dominates in Central and South America. AB negative is the rarest common blood type, found in roughly 1% of donors.
Why O Negative Is the Universal Donor
O negative red blood cells lack A antigens, B antigens, and the Rh D antigen. There’s simply nothing on the cell surface for a recipient’s antibodies to attack. This makes O negative blood safe to transfuse to anyone in an emergency when there’s no time to determine the patient’s type. It’s the default blood used in trauma situations and ambulances.
On the receiving end, AB positive is sometimes called the universal recipient. Because AB-positive individuals carry both A and B antigens plus the Rh D antigen, their immune system won’t attack any combination of those markers on donor red blood cells. In practice, the full compatibility chart looks like this: type A positive can receive from A+, A-, O+, and O-. Type O negative can only safely receive O negative. Type B- can receive B- and O-. Cross-matching in the lab confirms compatibility before any non-emergency transfusion.
Beyond ABO and Rh: Minor Blood Groups
ABO and Rh get all the attention, but your red blood cells carry markers from dozens of other blood group systems. The most clinically relevant include the Kell, Kidd, and Duffy systems. These rarely matter for routine transfusions, but they become important for people who receive frequent transfusions or during pregnancy.
The Duffy system illustrates why. If you lack a specific Duffy antigen and receive blood that carries it, your immune system can develop antibodies against it. Those antibodies persist, and a future transfusion with the same mismatch can cause a delayed reaction where your body destroys the donated red blood cells. The same mechanism can cause problems in pregnancy if a mother develops antibodies against antigens inherited from the father. For patients with known antibodies against minor blood group antigens, blood banks specifically select antigen-negative units.
The Bombay Phenotype: A Rare Exception
Standard blood typing occasionally misses an extremely rare variant called the Bombay phenotype. People with this blood type lack a foundational molecule called the H antigen, which is the precursor that A and B antigens are built on. Without the H antigen, neither A nor B antigens can form on red blood cells, so these individuals look like type O on routine testing.
The difference shows up during antibody screening. Bombay phenotype individuals carry anti-A, anti-B, and anti-H antibodies, meaning they react against all standard blood types, including O. They can only safely receive blood from other people with the Bombay phenotype. The condition is caused by mutations in a gene called FUT1 and is inherited in a recessive pattern, requiring both parents to carry the variant. It’s most commonly found in parts of South Asia, but it remains exceptionally rare worldwide.