During an antibody titration, which pattern indicates a prozone reaction?

• A. Prezone reaction
• B. Prozone reaction
• C. Postzone reaction
• D. Incomplete complement inactivation

Answer: (B)

In this titration, the prozone pattern shows up when there is antibody excess relative to the antigen. The undiluted or low-dilution tubes exhibit little to no agglutination because the abundant antibodies saturate the available epitopes and prevent cross-linking required to form visible lattice structures. As you dilute the serum, antibody concentration falls into the optimal range for lattice formation, and agglutination appears in the later tubes. This shift from negative (or very weak) to positive with dilution is the hallmark of a prozone reaction, indicating that an initially high antibody titer was masking the true reactivity.