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

In addition to the antigens present in the ABO System, red blood cells sometimes have another antigen, a protein called the Rh factor.

• If the Rh factor is present, the PERSON's blood group is RhD positive;
• If the Rh factor is absent, the PERSON is RhD negative.

This means that a PERSON 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-).

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