In serologic testing, which phenomenon is caused by antibody excess and is remedied by diluting the serum?

In serologic testing, which phenomenon is caused by antibody excess and is remedied by diluting the serum?

• A. Prozone effect
• B. Postzone effect
• C. Both
• D. Neither

Answer: (A)

Cross-linking of antibodies to antigens to form visible lattices is required for agglutination to be observed. When there is antibody excess, the prozone effect occurs: antibodies saturate all available antigen binding sites without forming the bridging structures needed to link many particles together, so little or no agglutination is seen and the test can falsely appear negative. Diluting the serum lowers the antibody concentration relative to antigen, allowing antibodies to bind in a way that bridges multiple antigen molecules and produces the lattice needed for a positive reaction. The alternative, where antigen is in excess, is the postzone phenomenon, which is resolved by adjusting antigen or serum concentrations differently, not simply by dilution of serum.

Cross-linking of antibodies to antigens to form visible lattices is required for agglutination to be observed. When there is antibody excess, the prozone effect occurs: antibodies saturate all available antigen binding sites without forming the bridging structures needed to link many particles together, so little or no agglutination is seen and the test can falsely appear negative. Diluting the serum lowers the antibody concentration relative to antigen, allowing antibodies to bind in a way that bridges multiple antigen molecules and produces the lattice needed for a positive reaction. The alternative, where antigen is in excess, is the postzone phenomenon, which is resolved by adjusting antigen or serum concentrations differently, not simply by dilution of serum.