C1q is an important recognition protein in the complement system,which is a major protein cascade in the innate immune system.Upon recognition of a target by C1q,the target is marked for opsonization and destruction.C1q recognizes many pathogenic pattems directly,but an important target is the Fc domain of antibodies binding to their antigen.In this paper,the curvature-dependence of the interaction between IgG and C1q is studied by surface plasmon resonance and quartz crystal microbalance.IgG is organized in similar surface coverage densities on flat polystyrene surfaces and polystyrene nanoparticles of different sizes,and the amount of C1q binding to the IgG is investigated.Nanoparticles in solution were found to aggregate upon incubation with IgG,and therefore a new technique utilizing nanoparticles binding to antifouling polymer brush functionalized surfaces was used to prepare surfaces with nanoparticles for measurements with surface plasmon resonance.Interestingly antigen-bound IgG at the curved surface of nanoparticles showed 5.6 times lower binding of C1q compared to at matched fiat surfaces.There was no significant difference between the binding at 100 and 200 nm polystyrene particles.These findings are important for designing drug delivery systems to evade the complement system.