Prozone phenomenon (false negative serological test result) is encountered in a case of secondary syphilis where VDRL appears negative at undiluted serum. After serial dilution, a reactive result is obtained at 1:16. What is the immunological mechanism of the prozone effect?
- A Antigen excess over antibodies causing immune complex solubilisation
- B IgM blocking IgG binding sites on treponemal antigens
- C Complement consumption in the undiluted serum preventing agglutination
- D Excess antibody relative to antigen — antibody excess prevents lattice formation and visible agglutination/precipitation ✓
Explanation
The prozone phenomenon occurs when antibody concentration is vastly in excess of antigen: each antibody molecule is surrounded by multiple antigen molecules (or vice versa), preventing the formation of large cross-linked antigen-antibody lattices needed for a visible precipitate or agglutinate. As the serum is diluted, the antibody:antigen ratio moves toward equivalence, allowing lattice formation and a visible reaction. In secondary syphilis (peak antibody production), this is a recognised pitfall; all negative VDRL results from clinically suspicious cases should be diluted and retested. This also occurs in high-titre malarial, HIV, and autoimmune specimens.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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