There are 4 main blood groups (types of blood) - A, B, AB and O. Your blood group is determined by the genes you inherit from your parents.
Each group can be either RhD positive or RhD negative, which means in total there are eight main blood groups.
Antibodies and antigens
Blood is made up of red blood cells, white blood cells and platelets in a liquid called plasma. Your blood group is identified by antibodies and antigens in the blood.
Antibodies are proteins found in plasma. They're part of your body's natural defences. They recognise foreign substances, such as germs, and alert your immune system, which destroys them.
Antigens are protein molecules found on the surface of red blood cells.
The ABO system
There are four main blood groups defined by the ABO system.
- blood group A - has A antigens on the red blood cells with anti-B antibodies in the plasma
- blood group B - has B antigens with anti-A antibodies in the plasma
- blood group O - has no antigens, but both anti-A and anti-B antibodies in the plasma
- blood group AB - has both A and B antigens, but no antibodies
Blood group O is the most common blood group. Almost half of the UK population (48%) has blood group O.
Receiving blood from the wrong ABO group can be life threatening. For example, if someone with group B blood is given group A blood, their anti-A antibodies will attack the group A cells.
This is why group A blood must never be given to someone who has group B blood and vice versa.
As group O red blood cells don't have any A or B antigens, it can safely be given to any other group.
The NHS Blood and Transplant (NHSBT) website has more information about the different blood groups.
The Rh system
Red blood cells sometimes have another antigen, a protein known as the RhD antigen. If this is present, your blood group is RhD positive. If it's absent, your blood group is RhD negative.
This means you can be one of eight blood groups:
- A RhD positive (A+)
- A RhD negative (A-)
- B RhD positive (B+)
- B RhD negative (B-)
- O RhD positive (O+)
- O RhD negative (O-)
- AB RhD positive (AB+)
- AB RhD negative (AB-)
About 85% of the UK population is RhD positive (36% of the population has O+, the most common type).
In most cases, O RhD negative blood (O-) can safely be given to anyone. It's often used in medical emergencies when the blood type isn't immediately known.
It's safe for most recipients because it doesn't have any A, B or RhD antigens on the surface of the cells, and is compatible with every other ABO and RhD blood group.
The NHS Blood and Transplant (NHSBT) website has more information about the RH system.
Blood group test
To work out your blood group, your red cells are mixed with different antibody solutions. If, for example, the solution contains anti-B antibodies and you have B antigens on your cells (you're blood group B), it will clump together.
If the blood doesn't react to any of the anti-A or anti-B antibodies, it's blood group O. A series of tests with different types of antibody can be used to identify your blood group.
If you have a blood transfusion – where blood is taken from one person and given to another – your blood will be tested against a sample of donor cells that contain ABO and RhD antigens. If there's no reaction, donor blood with the same ABO and RhD type can be used.
Pregnancy
Pregnant women are always given a blood group test. This is because if the mother is RhD negative but the child has inherited RhD-positive blood from the father, it could cause complications if left untreated.
RhD-negative women of child-bearing age should always only receive RhD-negative blood.
Giving blood
Most people are able to give blood, but a lot fewer people than are needed to meet demand actually do.
You can usually donate blood if you:
- are fit and healthy
- weigh between 50kg (7st 12lb) and 158kg (25 st)
- are 17-65 years old
Read more information from the Welsh Blood Service.