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Blood Groups, Crossmatching and Transfusion Reactions Quiz
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What physical event produces visible agglutination of red cells in a positive blood grouping test?
- A Antibodies activate complement, which lyses red cells You picked ✕
- B Free antibody molecules bind to each other and form a lattice that traps red cells You picked ✕
- C Antibodies cross-link antigen-bearing cells You picked ✓
- D Plasma removes the matching antigen You picked ✕
- E Platelets trap cells in a fibrin clot You picked ✕
Correct!
Incorrect
The answer is C: Antibodies cross-link antigen-bearing cells
Explanation
Why C is correct
See Grouping and Agglutination. Option C is correct because agglutination is the clumping of red cells that happens when antibodies cross-link antigen-bearing cells. In a grouping test, a known reagent antibody is mixed with red cells (or known reagent red cells with the patient's plasma). If the cells carry the matching antigen, the antibodies bind to antigens on neighbouring cells and join several cells into a visible lattice, a net-like clump. A clump in the well means the reagent antibody found its antigen. The NHS describes the same idea: if a solution containing anti-B antibodies is mixed with red cells that carry B antigen, the blood clumps together. The blood group is read from the pattern of clumping, and no clump means the antigen is absent. Agglutination is not haemolysis, because the cells stay whole, and it is not clotting, because no platelets or fibrin are involved. Memory aid: antibodies are bridges, and a clump is what you see when the bridges join the cells together.
- Agglutination: clumping of red cells when antibodies cross-link cells that carry the matching antigen.
- Grouping test: a clump means the reagent antibody found its antigen, and no clump means the antigen is absent.
- Not the same as: haemolysis or clotting, because the cells stay whole and no platelets or fibrin are involved.
Why the other options are incorrect
- A. Antibodies activate complement, which lyses red cells: Complement activation can lyse red cells, but lysis releases haemoglobin and does not produce visible clumping. Agglutination requires antibodies to cross-link intact antigen-bearing cells. Your answer
- B. Free antibody molecules bind to each other and form a lattice that traps red cells: Antibodies do not bind to each other to form a lattice; they bind specifically to antigens on the red-cell surface. The lattice is formed when one antibody bridges antigen sites on neighbouring cells. Your answer
- D. Plasma removes the matching antigen: The antigen is fixed to the red-cell surface, and antibodies bind to it rather than removing it. If the antigen were removed, there would be nothing left for the antibody to cross-link. Your answer
- E. Platelets trap cells in a fibrin clot: Platelets and fibrin belong to blood clotting, which is a different process. Agglutination needs only antibody and antigen-bearing red cells. Your answer
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