Red blood cells of which blood group will not be agglutinated by anti-A, B serum?

• A. A
• B. B
• C. AB
• D. O

Answer: (D)

In ABO typing, red cells react with antisera only if the corresponding antigen is present on the cell surface. An anti-A, anti-B serum contains antibodies against both A and B antigens. If a cell lacks both A and B antigens—as is the case with type O blood—there is nothing for the antibodies to bind to, so no agglutination occurs. Therefore, red blood cells from type O will not be agglutinated by anti-A, anti-B serum. By contrast, type A has A antigen and will react with anti-A, type B has B antigen and will react with anti-B, and type AB has both antigens and will react with both, leading to agglutination.