What a blood type is
A blood type, or blood group, is a label based on certain molecules found on the surface of red blood cells. These molecules are called antigens. Your immune system treats the antigens it was born with as “self,” and it generally makes antibodies against versions it does not have. That simple fact is the reason blood has to be matched before a transfusion.
Dozens of blood group systems are recognized. The two that matter most in everyday medicine are the ABO system and the Rh system.
The ABO system
The ABO groups were identified around 1900 by the Austrian scientist Karl Landsteiner, who noticed that mixing blood from some people caused the cells to clump together. The system has four main groups.
| Group | Antigens on red cells | Antibodies in plasma |
|---|---|---|
| A | A | anti-B |
| B | B | anti-A |
| AB | A and B | neither |
| O | none | anti-A and anti-B |
A person with group A blood has A antigens and carries antibodies against B. If group B blood were given to them, those antibodies would attack the donor cells, which can cause a dangerous reaction. The same logic applies to every combination in the table.
The Rh system
The Rh system is named after the rhesus monkey, which was used in early research. Its most important antigen is called D. People who have it are Rh positive, and those who lack it are Rh negative. Unlike ABO, Rh-negative people do not usually have anti-D antibodies from birth. They typically make them only after being exposed to Rh-positive blood, for example through a mismatched transfusion or sometimes during pregnancy.
Combining the two systems gives the familiar labels such as A positive or O negative. The relative frequency of each varies considerably among populations around the world, so there is no single global pattern.
Who can give blood to whom
For red blood cells, the donor’s cells should not carry antigens that the recipient’s antibodies will attack.
- Group O red cells have no A or B antigens, so they can usually be given to people of any ABO group. For this reason O negative blood is often described as the “universal donor” type and is used in emergencies when there is no time to test.
- People with group AB have no anti-A or anti-B antibodies, so they can generally receive red cells of any ABO group and are sometimes called “universal recipients.”
- Rh-negative recipients are generally given Rh-negative blood, while Rh-positive recipients can usually receive either.
These descriptions are simplifications. Hospitals do not rely on them alone. Before a transfusion the patient’s blood is typed, screened for unexpected antibodies and cross-matched with the donor blood. Other blood group systems can occasionally cause reactions, which is one reason testing is thorough. Plasma, the liquid part of blood, follows nearly the reverse pattern, because what matters there is the donor’s antibodies.
Why it matters in pregnancy
If an Rh-negative woman carries an Rh-positive baby, some of the baby’s blood can occasionally mix with hers, particularly around birth. Her immune system may then produce anti-D antibodies, which in a later pregnancy could cross the placenta and harm an Rh-positive baby’s red cells. This is well known to doctors, and routine screening and a preventive treatment given during and after pregnancy have made the problem much less common in places with good care. The details are managed by medical teams, and anyone who is pregnant should follow the advice of their own health professionals.
How blood types are inherited
Blood group is determined by genes, with one version of the gene coming from each parent. The A and B versions are dominant over O, and A and B are equally strong when they appear together. This means two group A parents can have a group O child if each carries a hidden O version. Patterns like this follow the ordinary rules of inherited traits and can help explain how family members end up with different groups.
Common questions
Blood type is not known to determine personality, and claims linking it to character or to diets are not supported by good evidence. Some associations between blood groups and the risk of certain diseases have been reported in research, but they are generally small and not something to act on without medical advice.
Knowing your group can be useful if you want to donate, and donation centers can usually tell you. Because blood supply depends on volunteers, and matching is so important, donors of all groups are valued. Medical matters about your own blood should be discussed with a qualified professional, as this article offers general education only.