Abstract: A kinetic Monte Carlo simulation is performed to investigate the gelation regions for the antibody-antigen system of 〖[Ag]〗_3-〖[Ab]〗_2 type, involving the semi-batch and nonequal reactivity. Specifically, the gelation regions consisting of critical and maximum conversions for various molar ratios of epitopes to paratopes are presented. Furthermore, the increments of critical conversions between two successive feedings are obtained to determine the equivalent zone, which ranges from 1 to 1.5 for the present system. It is shown that when the growth of antibody-antigen complexes of larger sizes takes priority, the smallest variance in the increments of critical conversions can be found. These results signify that, the agglutination reactions under the semi-batch mode can be employed to the quantitative analysis of the immune responses. Conversely, if the growth of complexes of small sizes dominates in the system, then the corresponding agglutination reactions are only suitable for qualitative immunoassay. An attempt was made to elucidate the impacts of relevant factors on the gelation process of the system, thereby providing useful clues for a quantitative immunoassay and relevant targeted drug therapies.

Key words: Antibody-antigen complex, Gelation region, Semi-batch, Nonequal reactivity, Kinetic Monte Carlo simulation