A laboratory technologist performs a serological test for syphilis and observes a false-negative result in a patient with a very high antibody titre. When the serum is diluted 1:100, the test becomes positive. This phenomenon is best described as which of the following?
- A Zone of equivalence
- B Cross-reactivity
- C Hook effect (prozone phenomenon) ✓
- D Epitope spreading
Correct answer: C. Hook effect (prozone phenomenon)
Explanation
The prozone effect (hook effect) occurs when very high antibody concentrations saturate all antigen epitopes, preventing cross-linking and visible lattice formation in agglutination or precipitation assays. Dilution reduces antibody concentration to the zone of equivalence, allowing proper lattice formation and a positive reaction. This is classically seen in syphilis serology and certain immunoassays.
Reference: Basic and Clinical Immunology by Stites, 2nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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