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 world’s 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-)
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