There are four 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 defense. They recognize 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
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-)
- RhD positive (O+)
- RhD negative (O-)
- AB RhD positive (AB+)
- AB RhD negative (AB-)
Red blood cell compatibility table
The following table shows the compatibility among all blood types when it comes to blood transfusion.
Source: NHS, Wikipedia