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Definition of Blood groups

Your blood group can be identified by the existence or absence of antigens and antibodies. Protein molecules called antigens are located on the surface of red blood cells and antibodies are found in the plasma. Antibodies are your blood’s natural defence against any foreign antigens in your blood.

• Blood group A has A antigens in its red blood cells and anti-B antibodies in its plasma.
• Group B has B antigens and anti-A antibodies in its plasma. 
• Group O blood has no antigens but both anti-A and anti-B antibodies. It is the most common blood group in the UK.
• Group AB cells have both A and B antigens but no antibodies, as it would destroy itself.

The red blood cells can also have another protein called the rhesus factor, which is another antigen. If this antigen is present you are rhesus-positive and if it is absent you are rhesus-negative. This means that you can be one of eight blood groups: A positive, AB negative, O positive, and so on. 

Around 85% of the UK population is rhesus positive.

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