All blood types carry out the same life-sustaining functions, such as delivering oxygen throughout the body and helping your immune system fight infections. However, not all blood cells have the same combinations of surface proteins (antigens) and antibodies; it’s those unique combinations that help determine a person’s blood type.

Both A and B antigens play a significant role in blood typing, which in combination with specific antibodies create four major blood groups (A, B, AB, and O [has neither the A or B surface protein]), which is known as the ABO blood system.

The third antigen that influences blood type is called Rh, making your blood type either positive when it’s present or negative when it’s absent, breaking the four blood groups down into two subgroups (positive and negative) or eight blood types in total. (A+, A-, B+, B-, AB+, AB-, O+, O-). O+ is the most common blood type, AB- is the rarest.

This test uses the ABO system to determine blood type by measuring the combination of A and B antigens and specific antibodies that correspond to the four blood groups (A, B, AB, and O). The test also detects the presence or absence of Rh antigen to determine if your blood type is positive or negative.