What is the ABO blood group system and the Rh factor?

• A. Rh system determines A, B, AB, O antigens.
• B. ABO determines A, B, AB, O antigens; Rh system determines presence (+) or absence (-) of the D antigen
• C. ABO and Rh are unrelated
• D. Rh determines D antigen only; ABO has no role

Answer: (B)

The main idea here is understanding two separate blood group systems used to determine transfusion compatibility: the ABO system and the Rh system. The ABO system defines which carbohydrate antigens—A, B, both (AB), or neither (O)—are present on the surface of red blood cells. Your plasma contains antibodies against the ABO antigens you do not express, which is why giving someone the wrong ABO type can cause a transfusion reaction.

The Rh system is a separate factor based on the presence or absence of the D antigen on red cells. If the D antigen is present, the blood type is Rh positive; if it is absent, it is Rh negative. Together, these two systems yield the common eight phenotypes (A+, A-, B+, B-, AB+, AB-, O+, O-), and both aspects are considered in transfusion decisions.

So, the correct statement correctly links ABO to A, B, AB, and O antigens on red cells, and identifies Rh as determining the presence or absence of the D antigen. The other options misstate the roles: the Rh system does not determine ABO antigens; ABO and Rh are not completely unrelated in clinical practice, and ABO does play a role alongside Rh in compatibility.