Antibody multivalency effects in the direct binding model for vesicle immunolysis assays

Abstract

An extension of a previous model of liposome-based immunoassays is presented which incorporates the effects of antibody multivalency in the binding process. Equations based on the distribution of vesicles having both mono- and divalently bound species show the quantitative relationships of the experimental parameters, including vesicle concentration, antigen density on vesicle surfaces, antibody concentration, and antibody affinity (both for the initial binding step and for the subsequent cross-linking step). It is found that in the case of low antibody concentration, the multivalent model can be cast in the form of the previously described monovalent model, replacing the association equilibrium constant with an effective equilibrium constant which is found to depend linearly on the lateral antigen density and on the valency of the binding antibody. Comparisons to certain experiments are made using this more realistic model of complement-mediated vesicle immunoassay. For the case of IgM binding, it is estimated that as few as 1000 antibody molecules can be detected in a typical lytic assay, representing a significant increase in sensitivity over previous predictions.