Your blood type is a label that describes specific molecules sitting on the surface of your red blood cells. These molecules, called antigens, determine which blood your body accepts as safe and which it attacks as foreign. There are eight common blood types, each defined by two systems: the ABO group (A, B, AB, or O) and the Rh factor (positive or negative). Together, they produce types like O-positive or AB-negative.
The ABO System
The letter in your blood type refers to a specific antigen, a sugar-based molecule, coating 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.
Here’s where it gets interesting: your immune system produces antibodies that target whichever antigen you don’t have. If you’re type A, your plasma contains antibodies against the B antigen. If you’re type B, you have antibodies against A. Type O is the most aggressive in this regard, carrying antibodies against both A and B. Type AB produces no antibodies against either, making it the most accepting of outside blood.
This is why blood type matters most during transfusions. If a type A person receives type B red blood cells, their anti-B antibodies latch onto those foreign cells and destroy them. That reaction can be life-threatening.
What the Positive or Negative Means
The second half of your blood type, the positive or negative, refers to a protein called the Rh factor (also known as the D antigen). If your red blood cells carry this protein, you’re Rh-positive. If they don’t, you’re Rh-negative. About 85% of people are Rh-positive.
Rh status is especially important during pregnancy. When an Rh-negative mother carries an Rh-positive baby, her body can recognize the baby’s blood cells as foreign and build antibodies against them. Those antibodies can cross the placenta and attack the baby’s red blood cells, causing a form of anemia where the cells are destroyed faster than the baby can replace them. In severe cases, this leads to liver failure, heart failure, or stillbirth. A medication called RhIg, commonly known by the brand name RhoGAM, prevents this by stopping the mother’s body from producing those antibodies in the first place. It’s given around 28 weeks of pregnancy and again within 72 hours of delivery.
How You Inherit Your Blood Type
You inherit one allele of the blood type gene from each parent, and the combination of those two alleles determines your type. The A allele produces the A antigen, the B allele produces the B antigen, and the O allele produces a nonfunctional protein that creates no surface antigen at all.
A and B are codominant, meaning if you inherit one of each, both antigens show up on your cells and your blood type is AB. The O allele is recessive. You need two copies of it to be type O. Someone with type A blood could carry either two A alleles or one A and one O. This is why two parents who are both type A can have a type O child: if each parent carries a hidden O allele, the child has a 25% chance of inheriting both.
How Common Each Type Is
Blood type distribution varies by ethnicity, but in the U.S. population the breakdown looks like this:
- O-positive: 37.4% (the most common)
- A-positive: 35.7%
- B-positive: 8.5%
- AB-positive: 3.4%
- O-negative: 6.6%
- A-negative: 6.3%
- B-negative: 1.5%
- AB-negative: 0.6% (the rarest common type, about 1 in 167 people)
Beyond these eight, there are extremely rare variants. The Bombay phenotype, for instance, lacks the foundational molecule (called the H antigen) that A and B antigens are built from. People with this phenotype look like type O on a standard test, but they also produce antibodies against that H antigen, which regular type O blood carries. The result is that they can only safely receive blood from other people with the Bombay phenotype, typically pulled from rare frozen inventories.
Who Can Donate to Whom
For red blood cell transfusions, the key rule is that your body will reject any antigen it doesn’t already carry. Type O-negative red blood cells carry no A, B, or Rh antigens, so they won’t trigger an immune reaction in anyone. That’s why O-negative is the universal red cell donor and the type used in emergencies when there’s no time to test the patient.
On the receiving end, type AB-positive can accept red blood cells from any type because the AB-positive immune system doesn’t produce antibodies against A, B, or Rh antigens.
Plasma transfusions flip these rules. Since plasma contains antibodies rather than antigens, you want plasma that won’t attack the recipient’s red blood cells. Type AB plasma contains no anti-A or anti-B antibodies, making it the universal plasma donor. Type O plasma, loaded with both types of antibodies, can only go to other type O recipients.
Here’s a quick reference for red blood cell compatibility:
- Type O: can receive red cells from O only
- Type A: can receive from A and O
- Type B: can receive from B and O
- Type AB: can receive from A, B, AB, and O
Blood Type and Disease Risk
Your blood type carries small but real differences in risk for certain conditions. The most consistent finding involves blood clots. People with type A, B, or AB blood are up to 1.5 times more likely to develop deep vein thrombosis, pulmonary embolism, and other clotting events compared to type O. Type O blood has naturally lower levels of certain clotting proteins, which appears to be protective.
A large genetic study published in the American Heart Association’s journals found that type A specifically was linked to modestly higher risk of heart failure (about 14% higher than type O), elevated cholesterol, and atherosclerosis. Type B showed a 13% higher risk of heart attack compared to type O. On the flip side, non-O blood types had a slightly lower risk of high blood pressure.
These differences are statistically real but relatively small in absolute terms. Your lifestyle, diet, and family history still have a far greater influence on cardiovascular health than your blood type. No one needs to change their medical care based on being type A instead of type O. But if you already have risk factors for clotting disorders, knowing your blood type adds a small piece of useful context.