Replacement therapy in hemophilia A with exogenous coagulation factor VIII (FVIII) often results in the development of FVIII-neutralizing antibodies, referred to as inhibitors. Despite of large number of studies on the functional properties of FVIII inhibitors, detailed physicochemical characterization of their interactions is not available. Here we studied the biophysical mechanism of the interaction between a human pathogenic antibody-BO2C11 and its target antigen-FVIII. Kinetic and thermodynamic analyses implied that this interaction is not accompanied by significant conformational changes in the proteins. The data also suggested that association of BO2C11 to FVIII is driven mainly by a hydrophobic effect. The protein electrostatics however played a decisive role in this association. Thus, a gradual increase in ionic strength resulted in a considerable increase in the association rate of binding of BO2C11 to FVIII. Such an ionic strength-dependency is uncommon for other antibody-antigen interactions. Our data suggest that electrostatic effects observed for BO2C11-FVIII association may arise from high-energy penalty of desolvation of the charged residues at the binding interfaces. We hypothesize that untypical ionic strength dependence of association of BO2C11 to FVIII reflects the nature of the recognized epitope, namely a molecular surface involved in the binding of FVIII to phospholipids. The presented data provide mechanistic information about FVIII neutralization by an inhibitory antibody and also contribute to the understanding of the general mechanisms of antibody-antigen interactions. © 2009 Elsevier Ltd. All rights reserved.