Which of the following is true about the MNSs blood group system?

• A. It includes the U antigen
• B. It includes Lea as a component
• C. It is the same as the Lewis system
• D. It has no clinically significant antibodies

Answer: (A)

The MNSs system includes antigens on two red cell glycoproteins: M and N on glycophorin A, and S, s, and U on glycophorin B. The U antigen is carried by glycophorin B and is a high-incidence antigen, meaning most people have it. The rare U-negative phenotype occurs when GYPB is absent or altered, and individuals without U can form anti-U if exposed to U-positive red cells, which can cause transfusion reactions. That’s why the statement that the MNSs system includes the U antigen is correct and clinically important.

Lea is a Lewis system antigen, not part of MNSs, so that statement is not true. The MNSs system and the Lewis system are distinct. And while the MNSs system can produce antibodies, anti-U in particular is clinically significant, so saying there are no clinically significant antibodies would be incorrect.