Blood types explained: Understanding A, B, O, and the Rhesus factor

Wondering what your blood type is and what that means? It's all less complicated than you might think.

Blood types have two parts: an ABO group (A, B, AB or O) and a positive or negative RhD status. Together, these give the eight common blood types, such as A positive (A+) and O negative (O-). Your blood type helps determine which donated blood you can safely receive and can affect the care you need during pregnancy.

These letters and signs describe markers called antigens on your red blood cells. Your ABO group reflects which A or B markers you have; positive means the RhD marker is present, while negative means it is absent. You inherit your blood group from your parents.

At a glance

• Your blood type combines A, B, AB or O with a positive or negative RhD status. These labels describe markers on your red blood cells, not how healthy you are.

• O positive is the most common blood type among NHS Blood and Transplant donors. AB negative is the least common of the eight main types.

• O-negative red cells are compatible with all eight main blood types, making them useful in emergencies. Donation services need donors from every group.

• Being RhD negative can affect pregnancy care. Your maternity team checks your blood group and antibodies to identify any extra care you or your baby may need.

• Routine blood tests don’t usually reveal your blood type. You can learn it through blood donation or use a Blood Type Rapid Home Test Kit, which uses a finger-prick sample.

• You don’t need to know your blood type to receive emergency treatment. Hospitals carry out their own checks, and a home result cannot replace testing for transfusions or pregnancy care.

Below, we explain what each blood type means, why it matters and how to find out yours.

What do A, B, AB and O mean?

The ABO system sorts blood into four groups according to the A and B markers on red blood cells. These markers are called antigens.

• Group A has A antigens.
• Group B has B antigens.
• Group AB has A and B antigens.
• Group O has neither A nor B antigens.

Your blood also contains antibodies: proteins in the liquid part of blood, called plasma, that help your immune system recognise foreign substances.

In the ABO system, group A blood normally contains anti-B antibodies, while group B contains anti-A. Group O has both. Group AB has neither anti-A nor anti-B antibodies.

That matters when someone needs a transfusion. For example, if a person with group A blood receives group B red cells, their anti-B antibodies can attack those cells. Matching the blood protects against this reaction.

What do positive and negative mean?

The Rhesus system supplies the second part of your blood type. For the positive or negative label, the marker that matters is RhD.

If your red blood cells have the RhD antigen, you’re RhD positive. If they don’t, you’re RhD negative. You may see this written as Rh positive, Rh negative, RhD+ or RhD-.

The sign describes that marker, rather than rating your health. For example, A positive and A negative blood both have A antigens; the difference is the RhD antigen.

The eight main blood types

How can you find out your blood type?

You need a test to find out your blood group type. Having had blood tests before doesn’t necessarily mean someone has checked it: the NHS says GPs do not routinely test blood groups.

If you’ve had blood grouping as part of previous care, ask your healthcare team if they have a recorded result.

You can also learn your group after your first blood donation. You don’t need to arrive knowing it. Before booking, check the donation service’s eligibility guidance, which covers health, medicines and recent travel.

For personal information, a home blood group kit offers another option. A Blood Type (A, B, O, Rhesus) Rapid Home Test Kit uses a finger-prick sample to identify your ABO and RhD group. Follow the supplied instructions and result guide.

A home result does not replace the clinical blood testing needed for transfusions or pregnancy care. If your result is unclear, ask for advice rather than guessing your group.

What does your blood type mean for donation?

Blood donation services need a range of blood groups because patients need compatible blood. A common group can still be in high demand.

O-negative red cells are particularly useful in emergencies because they are compatible with all eight main ABO/RhD types. That is why people with O-negative blood are often called universal red cell donors.

This refers to red cells. Different blood components, such as plasma, have different compatibility rules.

Read our blog on what to expect when you donate blood for the first time to understand the link with blood groups in more detail. 

What does RhD negative mean during pregnancy?

If you’re RhD negative, your maternity team will explain what that means for your pregnancy. A negative result on its own does not mean your baby has a problem.

The concern arises if your baby is RhD positive and your immune system develops antibodies against RhD-positive red cells. Those antibodies can cross the placenta and damage the baby’s red blood cells. This can cause haemolytic disease of the fetus and newborn, sometimes called rhesus disease.

Antenatal care includes blood group and antibody screening. Your team may also offer a blood test to predict your baby’s RhD group. When appropriate, anti-D injections help prevent you from developing the antibodies. If you already have them, your team will discuss monitoring and care.

Ask your midwife to explain your recorded result and any next steps. A home blood group test cannot provide the antibody screening your maternity team needs.

Do you need to know your blood type in an emergency?

You don’t need to know your blood type for emergency staff to treat you. Hospitals use blood grouping and compatibility checks to select suitable blood, rather than relying on a result you remember or carry with you.

When blood is needed before the patient’s group is known, clinicians can use O-negative red cells. The absence of A, B and RhD antigens makes them compatible across the eight main types.

Knowing your group can satisfy your curiosity and help you understand medical conversations. It doesn’t replace those hospital checks.

Which blood types are most common?

O positive is the most common of the eight main types among NHS Blood and Transplant donors. AB negative is the least common.

The table below shows the service’s donor figures, dated February 2026. These describe donors, rather than the whole UK population, and the percentages are rounded.

Questions about blood types

Will a routine blood test show my blood group?

Usually, no. A full blood count looks at your blood cells and measures such as haemoglobin. Blood grouping is a separate test, so a previous blood test does not automatically give you an ABO or RhD result.

Can O-positive blood be given to everyone?

No. O-positive red cells are compatible with O+, A+, B+ and AB+ recipients under the usual ABO/RhD rules. People with O-positive blood can receive O-positive or O-negative red cells. The RhD marker is why O positive and O negative have different uses in transfusions.

Which blood type is the universal recipient?

AB positive is often called the universal red cell recipient because it is compatible with red cells from all eight main ABO/RhD groups. Hospitals still carry out the appropriate compatibility checks before a transfusion.