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The ABO System is a classification for CLINICAL INVESTIGATION RESULT ITEM.

The ABO System is a system of 4 basic types into which human blood may be classified according to the presence or absence of particular antigens:

• A - Blood group A has A antigens in its red blood cells and anti-B antibodies in its plasma;
• B - Blood group B has B antigens and anti-A antibodies in its plasma;
• O - Blood group O blood has no antigens but both anti-A and anti-B antibodies
• AB - Blood group AB has both A and B antigens but no antibodies, as it would destroy itself.

For further information on the ABO System, see the NHS Choices website.