Question

Prozone phenomenon is seen with?

• A. Same concentration of antibody and antigen
• B. Antigen excess to antibody
• C. Antibody excess to antigen
• D. Hyperimmune reaction

Hint:

A false negative from too much antibody at low dilutions; dilute the serum and it turns positive.

Answer 1

The Correct Option is C

Step 1: The prozone phenomenon is a problem in agglutination and precipitation serological tests. It refers to a false negative result that occurs not because antibody is absent but because the proportions of reactants are wrong.

Step 2: Optimal lattice formation (visible clumping or precipitation) needs antigen and antibody in roughly equivalent amounts (the zone of equivalence). When antibody is in great excess compared with antigen, each antigen site gets coated by single antibody molecules and stable cross-linked lattices cannot form, so no visible reaction is seen. This false negative due to antibody excess is the prozone phenomenon. Hence option c is correct.

Step 3: When antigen is in excess (option b), a similar false negative can occur but this is termed the postzone phenomenon, not prozone. Equivalent concentrations (option a) give the strongest positive reaction, the opposite of prozone.

Step 4: Diluting the serum to reduce antibody concentration restores the visible reaction, confirming that antibody excess causes the prozone.