Mixed-field reaction with anti-A reagent and a negative reaction with anti-A1 lectin indicates which A subgroup?

• A. A3
• B. A1
• C. A2
• D. Ax

Answer: (A)

A3 is a red blood cell A-subgroup that often shows mixed-field agglutination with anti-A due to partial or uneven expression of the A antigen. Dolichos biflorus (anti-A1 lectin) binds the A1 epitope; if cells lack A1, the lectin test will be negative. The combination of a mixed-field reaction with anti-A and a negative anti-A1 lectin result is a classic pattern for A3, since these cells express A antigen without the A1 epitope. By contrast, A1 would typically give a strong anti-A reaction and a positive anti-A1 reaction, and A2 usually lacks A1 and does not show the characteristic mixed-field pattern. Ax can be variable, but the described pattern most strongly fits A3.