Your blood type is one of eight common categories determined by two things: which sugar molecules sit on the surface of your red blood cells (the ABO group) and whether a specific protein called the Rh factor is present (the positive or negative part). The eight types are A+, A-, B+, B-, AB+, AB-, O+, and O-. Most people don’t know their blood type off the top of their head, but it matters for transfusions, pregnancy, and organ donation.
How the ABO System Works
Every red blood cell is coated with sugar molecules called antigens. If your cells carry A antigens, you’re type A. If they carry B antigens, you’re type B. If they carry both, you’re type AB. If they carry neither, you’re type O.
Your immune system treats any antigen it doesn’t recognize as a threat. So your body produces antibodies against whichever antigen your own cells lack. If you’re type A, your blood contains anti-B antibodies. If you’re type B, you carry anti-A antibodies. Type O blood contains both anti-A and anti-B antibodies. Type AB blood contains neither, which is why AB individuals can receive red blood cells from any ABO group without triggering an immune reaction.
This is why mismatched transfusions are dangerous. If type A blood enters a type B person’s body, the anti-A antibodies in the recipient’s blood attack the donated cells, potentially causing a life-threatening reaction.
What Positive and Negative Mean
The “+” or “-” after your letter refers to the Rh factor, an inherited protein on the surface of red blood cells. If you have it, you’re Rh positive. If you don’t, you’re Rh negative. Around 85% of people are Rh positive.
Rh status is especially important during pregnancy. If a mother is Rh negative and her baby is Rh positive, her immune system can treat the baby’s blood cells as foreign invaders. This doesn’t usually cause problems in a first pregnancy, but her body may build up antibodies that attack the blood cells of a future Rh-positive baby. That immune response can cause severe anemia in the fetus, where red blood cells are destroyed faster than the body can replace them. In serious cases, it can be fatal. Fortunately, this is preventable with a medication given during pregnancy that stops the mother’s body from producing those antibodies in the first place.
Blood Type Distribution in the US
O+ is the most common blood type, found in about 37% of the US population. O- is far less common at roughly 7%, but it’s the universal red blood cell donor type because O- cells carry no A, B, or Rh antigens for a recipient’s immune system to attack. AB+ individuals, the rarest of the common eight, are considered universal recipients for red blood cell transfusions.
Distribution varies by ethnicity. Type B is more common in people of Asian and African descent, while type A is more prevalent in European populations. These differences reflect thousands of years of population genetics and migration patterns.
How You Inherit Your Blood Type
You get one ABO gene from each parent. A and B are codominant, meaning if you inherit one of each, both antigens show up on your cells and you’re type AB. O is recessive, so you need two copies (one from each parent) to be type O. This means a person who appears to be type A could carry either two A genes (AA) or one A and one O gene (AO). The same applies to type B: your genotype could be BB or BO.
This is why two parents who are both type A can have a type O child. If each parent carries a hidden O gene (AO), there’s a one-in-four chance their child inherits both O copies. Rh factor follows a similar pattern. Rh positive is dominant, so two Rh-positive parents can have an Rh-negative child if both carry one copy of the recessive negative gene.
Rare Blood Types Beyond the Big Eight
Beyond the standard ABO and Rh system, there are more than 300 known blood group antigens. Most are clinically insignificant, but a few rare types create serious challenges for transfusion medicine.
The Bombay phenotype is one of the most striking examples. People with this type lack the foundational molecule (called the H antigen) that A and B sugars are built on. Their blood looks like type O on a standard test, but it’s fundamentally different. They produce antibodies against the H antigen that virtually everyone else carries, making them incompatible with nearly all donated blood. Only another person with the Bombay phenotype can safely donate to them. It occurs in roughly one in 10,000 people in India and one in a million in Europe, so compatible blood often comes from rare frozen inventories.
How to Find Out Your Blood Type
There are several practical ways to learn your type. The most reliable is a lab blood test called ABO typing. A technician mixes a small sample of your blood with antibodies against A and B antigens and watches whether the cells clump together. Clumping with anti-A antibodies means you’re type A; clumping with anti-B means type B; clumping with both means AB; no clumping means type O. A second step, called back typing, confirms the result by mixing your plasma with known A and B cells. Rh typing works the same way, checking whether your cells react with anti-Rh antibodies.
You can request a blood type test through your doctor. Many people also learn their type when they donate blood, since the blood bank types every donation and provides results on your donor card. At-home blood typing kits are available as well. These use the same clumping principle with a finger prick sample and a card coated with dried antibodies. They’re generally accurate when done correctly, but a lab test is more reliable if you need a confirmed result for medical purposes.
If you’ve had surgery, given birth, or received a transfusion, your blood type is likely already in your medical records. Your doctor’s office can usually look it up without ordering a new test.