Most people know what their blood type is—or at least you should.
Your blood group is determined by genes that you inherit from your parents. It depends on substances in your blood:
- antigens – proteins found on the surface of red blood cells that cause antibodies to be produced
- antibodies – infection-fighting proteins found in plasma (the liquid part of blood) that are part of your immune system and attack specific antigens if they’re found in your body
The ABO system
Under this system, your blood may belong to one of four groups:
- A – you have A antigens on your red blood cells and anti-B antibodies (antibodies that attack B antigens)
- B – you have B antigens and anti-A antibodies
- AB – you have A and B antigens and no anti-A or anti-B antibodies
- O – you have no antigens, but both anti-A and anti-B antibodies
The Rh system
Red blood cells can also have another antigen called the rhesus factor (Rh factor).
Your blood can be:
- RhD positive (also called rhesus positive) – the antigen is present
- RhD negative (also called rhesus negative) – the antigen is not present