Your blood type is a label for the specific combination of proteins sitting on the surface of your red blood cells. These proteins, called antigens, determine how your immune system recognizes blood as “self” or “foreign.” There are eight common blood types, built from two classification systems layered together: the ABO group (A, B, AB, or O) and the Rh factor (positive or negative). The most common type worldwide is O positive, found in roughly 40% of people, while the rarest is AB negative, at less than 1%.
The ABO Group: A, B, AB, and O
The letters in your blood type refer to which antigens your red blood cells carry. Type A blood has A antigens. Type B has B antigens. Type AB has both. Type O has neither.
Your immune system produces antibodies against whichever antigens your own blood cells lack. If you’re type A, your plasma contains antibodies that attack B antigens. If you’re type B, you carry antibodies against A. Type AB blood has no antibodies against either antigen, which is why people with AB blood can receive red cells from any ABO group. Type O blood carries antibodies against both A and B antigens, meaning your immune system will react to any blood that isn’t also type O.
This antibody response is the reason blood type matters during transfusions. If you receive blood with antigens your body recognizes as foreign, those antibodies trigger an immune reaction that can destroy the donated red blood cells, potentially causing organ damage or death.
What Positive and Negative Mean
The “positive” or “negative” after your letter refers to the Rh factor, specifically a protein called the D antigen. If your red blood cells carry this protein, you’re Rh positive. If they don’t, you’re Rh negative. At a genetic level, most Rh-negative people simply have a deleted copy of the gene responsible for making the D protein.
Rh status matters less in everyday life than the ABO group, but it becomes critical in two situations: blood transfusions and pregnancy. Rh-negative patients should receive Rh-negative blood, while Rh-positive patients can safely receive either. This is why O negative is considered the universal red cell donor type. In emergencies when there’s no time to test a patient’s blood, O negative is the safest option because it carries no A, B, or D antigens for the recipient’s immune system to attack.
How You Inherit Your Blood Type
You inherit one allele of the blood type gene from each parent, giving you two copies total. Three versions of the gene exist: A, B, and O. The A allele produces an enzyme that builds A antigens on your red blood cells. The B allele produces an enzyme that builds B antigens. The O allele produces a nonfunctional protein that doesn’t build any surface antigen at all.
A and B are both dominant over O, but neither is dominant over the other. This creates some combinations that surprise people. If you inherit an A allele from one parent and an O from the other, your blood type is A, because the O allele contributes nothing. The same logic applies to B. But if you inherit an A from one parent and a B from the other, both alleles express themselves equally, giving you type AB.
This means two parents who are both type A can have a child with type O, if each parent carries a hidden O allele. It also means a type A parent and a type B parent could have children with any of the four blood types, depending on what second allele each parent carries. Rh factor follows a similar pattern: positive is dominant over negative, so two Rh-positive parents can have an Rh-negative child if both carry one copy of the negative variant.
Who Can Donate to Whom
Transfusion compatibility follows a straightforward set of rules based on which antigens the donor’s blood carries and which antibodies the recipient’s blood contains.
- Type O negative can donate red blood cells to anyone, making it the universal red cell donor. It carries no A, B, or D antigens.
- Type O positive can donate to any Rh-positive recipient (A+, B+, AB+, O+).
- Type A can donate red cells to type A and type AB recipients.
- Type B can donate red cells to type B and type AB recipients.
- Type AB positive can receive red blood cells from any type, making it the universal recipient. However, AB donors can only give red cells to other AB individuals.
- Type AB is the universal plasma donor, because AB plasma contains no anti-A or anti-B antibodies that could attack a recipient’s cells.
In every case, Rh-negative blood can go to either Rh-positive or Rh-negative recipients, but Rh-positive blood should only go to Rh-positive recipients. This is why blood banks are always especially short on O-negative and other Rh-negative types.
Rh Factor and Pregnancy
Rh incompatibility becomes a concern when an Rh-negative mother carries an Rh-positive baby. During delivery, or sometimes during pregnancy, small amounts of the baby’s blood can enter the mother’s bloodstream. Her immune system may recognize the D antigen as foreign and begin producing antibodies against it. This usually doesn’t affect the first pregnancy, but in subsequent pregnancies with Rh-positive babies, those antibodies can cross the placenta and attack the baby’s red blood cells. Before modern prevention, 14% of affected fetuses were stillborn, and half of live-born infants suffered death or brain injury.
A preventive treatment introduced in the 1970s changed this dramatically. Rh-negative mothers receive an injection of Rh immune globulin, which prevents their immune system from forming antibodies against the D antigen. Initially given only after delivery, this approach reduced the risk from roughly 13 to 16% down to about 0.5 to 1.8%. When doctors added a routine injection during pregnancy as well, the risk dropped further to 0.14 to 0.2%. Today, Rh incompatibility is almost entirely preventable with standard prenatal care.
How Common Each Blood Type Is
Blood type distribution varies significantly by ethnicity and geography, but global averages paint a general picture. O positive is by far the most common, averaging around 40% of the population and reaching as high as 75% in some regions. A positive is the second most common. B positive is more prevalent in South and East Asian populations than in European ones. AB positive is uncommon everywhere, and AB negative is the rarest type globally, averaging under half a percent.
Your blood type is determined at birth and never changes. Knowing it is useful not just for medical emergencies but for understanding potential pregnancy considerations and for making blood donation as efficient as possible. If you’ve never been typed, a simple blood draw at a doctor’s office or blood donation center will give you your result in minutes.