Immunomagnetic reduction assay is used to quantitatively detect bio-molecules. Many reports show that the to-be-detected bio-molecular concentration dependent reduction in the alternative-current (ac) magnetic susceptibility of a reagent is governed by the logistic function, which is a four-parameter function. One of the parameters relates to the increase in the rate of the magnetic reduction signal when the concentration of to-be-detected bio-molecules is increased. Theoretically, this parameter is attributed to the clustering associations between to-be-detected bio-molecules and labeling particles in the reagent. In an immunomagnetic reduction assay, the bioactive labeling particles are anti-body-functionalized magnetic nanoparticles. However, there is no detailed information about the effect of the clustering associations on this parameter. In this work, the clustering association is manipulated by controlling the concentrations of anti-body-functionalized magnetic nanoparticles in the reagent. The experimental results show that higher values for this parameter are obtained with concentrated anti-body-functionalized magnetic nanoparticles in the reagent. This implies that particle clustering is enhanced by an increase in the concentration of the bio-functionalized magnetic particles. It is also demonstrated that the particle clustering effect dominates the increased rate of the magnetic reduction signal.