The prozone phenomenon in syphilis serology occurs because:
- A Excess antigen saturates all antibody binding sites, preventing lattice formation
- B Non-specific IgM blocks access of specific IgG to the antigen
- C Excess antibody saturates all antigen epitopes, preventing cross-linking and lattice formation ✓
- D High-avidity IgG blocks low-avidity IgM from binding complement
Correct answer: C. Excess antibody saturates all antigen epitopes, preventing cross-linking and lattice formation
Explanation
The prozone phenomenon occurs when antibody is present in great excess relative to antigen; all antigen epitopes are occupied by separate antibody molecules, preventing the cross-linking necessary to form an immune lattice and causing a false-negative result. Diluting the serum resolves this. Antigen excess causing false-negative is called the post-zone phenomenon. In secondary syphilis with very high RPR titres, prozone can mask a positive result.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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