There are four main blood types (A, B, AB, and O), and when you factor in the Rh protein, the count doubles to eight. Those eight types are what matter for blood transfusions and what most people mean when they ask the question. But the full picture is more complex: scientists have identified over 300 blood group antigens across at least 33 recognized classification systems, making each person’s blood profile far more detailed than a simple letter and symbol.
The Eight Common Blood Types
Your blood type depends on two things: which sugar molecules (antigens) sit on the surface of your red blood cells, and whether you carry a protein called the RhD antigen. The ABO system sorts you into four groups based on antigens:
- Type A: has the A antigen
- Type B: has the B antigen
- Type AB: has both A and B antigens
- Type O: has neither A nor B antigen
Each of those four is then classified as positive or negative depending on the RhD antigen. If your red blood cells carry it, you’re positive; if they don’t, you’re negative. That gives us the eight types most hospitals use: A+, A-, B+, B-, AB+, AB-, O+, and O-.
How Common Each Type Is
Blood type distribution varies by ethnicity and region, but donor data from NHS Blood and Transplant gives a useful snapshot. O positive is the most common type by a wide margin, found in about 36% of donors. A positive comes next at 28%. Together, those two types account for nearly two-thirds of the population.
After that, the numbers drop quickly. O negative makes up about 14%, A negative about 8%, and B positive about 8%. B negative sits at roughly 3%. The AB types are the rarest of the common group: AB positive at around 2% and AB negative at just 1%.
Why Blood Type Matters for Transfusions
If you receive red blood cells with antigens your body doesn’t recognize, your immune system attacks them. That’s why matching blood type is critical. Type O negative red blood cells can be given to anyone because they carry no A, B, or RhD antigens for the recipient’s immune system to react to. That makes O negative the universal red cell donor type, and it’s the go-to in emergency rooms when there’s no time to test.
On the receiving end, AB positive individuals can accept red blood cells from any type, making them universal recipients. The rules flip for plasma transfusions: AB plasma is the universal donor because it contains no anti-A or anti-B antibodies that could attack the recipient’s cells, while O recipients can receive plasma from any type.
Beyond the Big Eight: Rare Blood Types
The ABO and Rh systems get nearly all the attention, but they’re just two of at least 33 blood group systems recognized by the International Society of Blood Transfusion, collectively accounting for over 300 individual antigens. Most of these rarely cause problems in transfusions, but a few create extraordinary challenges.
The Bombay phenotype (sometimes written as Oh) is one of the rarest. People with this type lack not only the A and B antigens but also a foundational antigen called H, which is normally present on virtually all red blood cells, including type O. Their immune system produces antibodies against A, B, and H antigens, meaning they can only receive blood from another person with the Bombay phenotype. Standard type O blood, which works for almost everyone else, is incompatible. The phenotype was first identified in Mumbai and occurs most frequently in parts of South Asia, though it remains extremely uncommon even there.
Another exceptionally rare type is Rh-null, sometimes called “golden blood.” People with Rh-null lack all antigens in the entire Rh system, not just the D antigen. Fewer than 50 people worldwide have been documented with it. Their red blood cells can theoretically be given to anyone with rare Rh types, making their donations extraordinarily valuable, but finding a match for them when they need a transfusion is nearly impossible.
How Blood Type Is Inherited
You inherit your blood type from your parents through a straightforward genetic pattern. For the ABO system, you receive one gene copy from each parent. The A and B versions of the gene are codominant, meaning if you get one of each, both express themselves and you end up with type AB. The O version is recessive, so it only shows up when you inherit it from both parents.
This is why two parents with type A blood can have a child with type O. Each parent could carry one A gene and one hidden O gene. If the child inherits the O gene from both, they’ll be type O. The Rh factor follows a similar logic: positive is dominant over negative. Two Rh-positive parents can have an Rh-negative child if both carry a recessive negative gene.
How Your Blood Type Is Determined
Blood typing in a lab involves two complementary tests. The first, called forward typing, mixes your red blood cells with solutions that react specifically with A and B antigens. If the cells clump together with the anti-A solution, you have the A antigen. If they clump with anti-B, you have B. If both cause clumping, you’re AB; if neither does, you’re O.
The second test, reverse typing, works from the other direction. It takes the liquid portion of your blood (serum) and mixes it with known A and B red blood cells. Your serum naturally contains antibodies against the antigens you don’t have, so the clumping pattern should mirror the forward test. When both tests agree, the result is confirmed. Rh typing only requires the forward test, checking whether your cells react with an anti-RhD solution.
Home blood typing kits use a simplified version of the same principle, applying drops of your blood to a card pre-treated with anti-A, anti-B, and anti-RhD reagents. They’re generally reliable for basic ABO and Rh typing, though hospitals always run their own tests before a transfusion.