Your blood type is determined by the genes you inherit from your biological parents, which dictate specific molecules on the surface of your red blood cells. In the lab, blood type is identified by mixing a blood sample with known antibodies and watching for a clumping reaction. The system most people are familiar with, the ABO and Rh grouping (like “O positive” or “A negative”), is just one piece of a much larger picture: scientists have identified 48 distinct blood group systems so far.
What Makes Blood Types Different
The differences between blood types come down to sugar molecules attached to the surface of red blood cells. These sugars act as markers, called antigens, that your immune system uses to distinguish “self” from “foreign.” In the ABO system, the key distinction is a single sugar variation. Type A cells carry one sugar arrangement, type B cells carry a slightly different one, type AB cells carry both, and type O cells carry neither.
Your immune system produces antibodies against whichever antigen you lack. If you have type A blood, your body makes antibodies against B. If you have type B, you make antibodies against A. People with type AB make no ABO antibodies at all, while people with type O make antibodies against both A and B. This is why transfusing the wrong blood type triggers a reaction: the recipient’s antibodies attack and destroy the donor’s red blood cells.
How Genetics Controls Your Blood Type
You inherit one ABO gene from each biological parent, giving you two copies. Three versions of the gene exist: A, B, and O. The A and B versions are codominant, meaning if you inherit one of each, both are expressed and you end up with type AB blood. The O version is recessive, so it only shows up when you inherit it from both parents.
This creates some combinations that surprise people. Two parents who both have type A blood can have a child with type O, because each parent may carry a hidden O gene alongside their A gene. Specifically:
- Type O requires two O genes, one from each parent.
- Type A results from two A genes or one A and one O.
- Type B results from two B genes or one B and one O.
- Type AB requires one A gene from one parent and one B gene from the other.
The Rh Factor: Positive vs. Negative
The “positive” or “negative” in your blood type refers to a separate protein on your red blood cells called the D antigen, part of the Rh blood group system. If your cells carry this protein, you’re Rh-positive. If they don’t, you’re Rh-negative. In most people of European descent, being Rh-negative means the gene responsible for making the D protein is simply deleted from their DNA. In people of African or Asian ancestry, the gene may still be physically present but contain mutations that prevent it from producing a functional protein.
The Rh factor 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 usually isn’t a problem in a first pregnancy, but those antibodies can attack fetal red blood cells in future pregnancies. To prevent this, Rh-negative pregnant individuals receive a protective injection during and after pregnancy that blocks the immune response before it starts. Newer approaches now use fetal genotyping from a maternal blood draw to determine whether the fetus is actually Rh-positive, avoiding unnecessary treatment when the baby turns out to be Rh-negative.
How Blood Type Is Tested in the Lab
Blood typing relies on a straightforward principle: when antibodies encounter their matching antigen, red blood cells clump together visibly. Labs use this reaction in two complementary tests to confirm a result.
First, the blood sample is spun in a centrifuge to separate red blood cells from the liquid plasma. In the forward test, your red blood cells are mixed with known antibodies against A, B, and D antigens in three separate reactions. If your cells clump when exposed to anti-A antibodies, you carry the A antigen. If they clump with anti-D, you’re Rh-positive. No clumping means that antigen is absent.
The reverse test works in the opposite direction. Your plasma (which contains your natural antibodies) is mixed with known type A and type B red blood cells. If your plasma causes type B cells to clump but not type A cells, you carry anti-B antibodies, confirming you’re type A. The forward and reverse 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 NHS Blood and Transplant in the UK, the approximate breakdown is:
- O positive: 36%
- A positive: 28%
- O negative: 14%
- B positive: 8%
- A negative: 8%
- B negative: 3%
- AB positive: 2%
- AB negative: 1%
These percentages vary significantly by ethnicity and geography. Type B is more common in South Asian and East Asian populations, while type O dominates in Central and South America. Rh-negative blood is most prevalent among people of European descent and relatively rare in East Asian populations.
Why Blood Type Matters for Transfusions
The antibody-antigen pairing creates strict rules about who can receive blood from whom. Type O negative is considered the universal donor for red blood cells because O cells carry no A, B, or D antigens, so there’s nothing for a recipient’s antibodies to attack. This is the type used in emergency rooms when there’s no time to test. Type AB positive is the universal recipient, because people with AB positive blood carry all three major antigens and produce no ABO antibodies, so they can tolerate red cells from any ABO/Rh combination.
Give someone the wrong type, though, and their antibodies will destroy the transfused cells rapidly. This can cause fever, kidney failure, and in severe cases, death. It’s why hospitals perform both forward and reverse testing and crossmatch donor blood against a recipient’s sample before every transfusion.
Rare Blood Types and Exceptions
Beyond the familiar eight ABO/Rh combinations, rare variants can complicate blood typing. One of the most notable is the Bombay phenotype, found in roughly 1 in 10,000 people in parts of Mumbai and about 1 in a million in Europe. People with this phenotype lack a foundational molecule called the H antigen, which is the building block that A and B sugars attach to. Without it, neither A nor B antigens can form on their red blood cells, regardless of what ABO genes they carry.
On routine testing, Bombay phenotype blood looks like type O. The difference shows up during antibody screening, where the sample reacts aggressively with all test cells, including type O cells. People with this phenotype can only receive blood from other Bombay donors, because their immune system produces antibodies against the H antigen that virtually everyone else carries. The genetic cause is a mutation in the gene responsible for making the enzyme that builds the H antigen on red blood cells.
The 48 blood group systems recognized by the International Society of Blood Transfusion encompass hundreds of individual antigens beyond A, B, and D. Most are clinically relevant only for people who need repeated transfusions or organ transplants, where exposure to even minor antigens can trigger antibody production over time. For everyday purposes, though, the ABO and Rh systems remain the two that matter most.