Blood types describe the specific molecules found on the surface of red blood cells. These molecules, called antigens, help determine how a person’s immune system recognizes their own blood and how it responds to blood from another person.
The two blood-group systems most familiar in the United States are the ABO system and the Rh system. Combining them produces the eight blood types commonly listed as A+, A−, B+, B−, AB+, AB−, O+, and O−.
Understanding blood types matters most when blood is being transfused or during pregnancy, when certain differences between a pregnant person and a fetus can sometimes cause medical problems.
The ABO blood group system
The ABO system divides blood into four main groups: A, B, AB, and O. The distinction depends on which antigens are present on the surface of red blood cells and which antibodies are found in the liquid portion of blood, called plasma.
- Type A blood has A antigens on red blood cells and naturally occurring anti-B antibodies in plasma.
- Type B blood has B antigens and anti-A antibodies.
- Type AB blood has both A and B antigens and does not normally have anti-A or anti-B antibodies in plasma.
- Type O blood has neither A nor B antigens and normally has both anti-A and anti-B antibodies.
An antigen is a molecule that the immune system can recognize. An antibody is a protein made by the immune system that can bind to a particular target. This distinction is important because transfusing incompatible blood can cause antibodies to attack the transfused red blood cells.
Why ABO compatibility matters
If a person receives red blood cells carrying an antigen that their plasma antibodies recognize as foreign, those antibodies can bind to the transfused cells. This can trigger destruction of the cells and, in severe cases, a dangerous transfusion reaction.
For red-blood-cell transfusions, the basic ABO pattern looks like this:
| Recipient’s type | Can generally receive red blood cells from |
|---|---|
| A | A, O |
| B | B, O |
| AB | A, B, AB, O |
| O | O |
This is a simplified compatibility guide. Real transfusions are based on laboratory testing and clinical protocols, and compatibility involves more than ABO alone.
The often-heard terms “universal donor” and “universal recipient” therefore need context. O negative red blood cells are commonly used when the recipient’s blood type is not yet known in an emergency because they lack A, B, and RhD antigens. AB positive people can generally receive red blood cells of any ABO/Rh type, although hospitals still perform appropriate compatibility testing whenever possible.
What the positive and negative signs mean
The plus or minus sign refers to the Rh blood group system, specifically whether red blood cells carry the RhD antigen.
If the D antigen is present, the blood type is Rh-positive. If it is absent, the person is Rh-negative.
For example:
- A+ means type A with the RhD antigen.
- A− means type A without the RhD antigen.
- AB+ means type AB with the RhD antigen.
- O− means type O without the RhD antigen.
The Rh system is considerably more complicated than a simple positive-or-negative distinction; it includes many antigens. But RhD is the one responsible for the familiar plus/minus classification.
Unlike the ABO system, people who are Rh-negative do not normally have anti-RhD antibodies simply because they lack the D antigen. They can develop them after exposure to Rh-positive blood, such as through a transfusion or pregnancy.
How blood type is inherited
Blood type is determined genetically. For the ABO system, a person inherits one ABO allele from each biological parent.
The three major ABO alleles are commonly represented as A, B, and O. A and B are codominant, meaning that when both are inherited, both traits are expressed. The O allele is generally recessive to A and B.
This produces several possible genetic combinations:
- AA or AO → type A
- BB or BO → type B
- AB → type AB
- OO → type O
The RhD trait is also inherited genetically, although its inheritance is more complex than the simplified positive/negative model sometimes used to explain it.
Because several genetic combinations can produce the same blood type, knowing one person’s blood type does not always tell you exactly which blood-type genes they carry.
Why people have different blood types
Blood-group differences are normal forms of human genetic variation. The genes responsible for blood-group antigens have changed over generations through mutation, inheritance, and natural selection.
These antigens are not simply labels used by doctors. They are molecules produced by genes and displayed on cell surfaces. The ABO gene, for example, encodes an enzyme that modifies a basic structure on red blood cells, producing the A or B antigen. With type O, a common form of the gene produces an enzyme that does not make the same modification, leaving the basic structure without an A or B antigen.
Blood-group patterns also vary among populations around the world. These differences reflect the complex history of human migration, ancestry, genetic variation, and selection.
Blood types and transfusions
Blood typing is essential before most transfusions because the immune system can react strongly to incompatible red blood cells.
Before giving blood, medical professionals typically determine the patient’s ABO and Rh type and perform crossmatching or other compatibility testing. Crossmatching checks whether the recipient’s blood reacts with the donor’s blood under laboratory conditions.
Blood products also matter. A unit of red blood cells is not the same as plasma or platelets. Their compatibility rules differ because plasma contains antibodies, while red blood cells carry the antigens that antibodies can target.
For example, AB plasma does not contain anti-A or anti-B antibodies and can therefore be useful for recipients of different ABO groups. Conversely, type O plasma contains both anti-A and anti-B antibodies, so its use requires careful consideration of the recipient’s red-cell antigens.
This is why statements such as “O is the universal donor” are incomplete without specifying which blood component is being transfused.
Blood types and pregnancy
Rh blood type can become important during pregnancy when an Rh-negative pregnant person carries an Rh-positive fetus.
During pregnancy or delivery, fetal blood cells can sometimes enter the pregnant person’s bloodstream. If the pregnant person becomes sensitized to the RhD antigen, their immune system can produce anti-D antibodies. In a later pregnancy with an Rh-positive fetus, these antibodies can cross the placenta and destroy fetal red blood cells.
This condition is called hemolytic disease of the fetus and newborn.
Modern prenatal care can usually prevent RhD sensitization with Rh immune globulin, a medication containing antibodies that help prevent the pregnant person’s immune system from becoming sensitized to RhD-positive red blood cells. Its use depends on the clinical circumstances.
ABO differences between a pregnant person and fetus can also sometimes contribute to newborn hemolysis, but ABO incompatibility is generally less problematic than RhD incompatibility.
Are blood types linked to health?
Blood type is primarily important for transfusion and certain pregnancy-related circumstances, but researchers have also found associations between ABO blood groups and various diseases and health outcomes.
These associations do not mean that having a particular blood type determines whether someone will develop a disease. Blood type is only one biological factor among many, and an association at the population level does not necessarily predict an individual’s health.
Blood-group antigens can influence biological processes beyond transfusion compatibility, including interactions involving blood-clotting proteins and cells. This helps explain why blood type can correlate with some health risks without being a simple cause of them.
For everyday health decisions, however, knowing your blood type generally does not replace established risk factors, screening recommendations, or medical advice.
There are more blood-group systems than ABO and Rh
ABO and Rh are the best-known systems, but they are not the only blood-group systems.
Red blood cells carry hundreds of different antigens. Internationally recognized blood-group systems include Kell, Duffy, Kidd, MNS, and others. Some of these antigens can also provoke immune reactions after transfusion or pregnancy.
This is particularly important for people who receive repeated transfusions. Their immune systems can become sensitized to less common blood-group antigens, making it necessary to identify specially matched blood.
So, although “blood type” is commonly used to mean one of the eight ABO/Rh combinations, a person’s full blood-group profile is much more complex.
How your blood type is determined
A blood-type test identifies antigens on red blood cells and, for ABO typing, checks the corresponding antibodies in the plasma.
In a typical ABO test, a blood sample is exposed to reagents containing anti-A and anti-B antibodies. If the red blood cells clump when exposed to one reagent, that indicates the corresponding antigen is present. Additional testing helps confirm the result.
RhD testing similarly determines whether the D antigen is present.
Your blood type is a stable inherited characteristic, although laboratory testing can occasionally require additional investigation when results are unusual or affected by medical circumstances. If blood is needed for a transfusion, the hospital does not rely solely on a person’s recollection of their blood type; it performs appropriate testing to establish compatibility.