Abstract: Antibodies against a virus can be detected in a sample using at least one reagent comprising at least one capture molecule immobilized to a particle, which reagent in the presence of such antibodies forms an anti-virus antibody-capture molecule complex, wherein the presence of said complex is qualitatively, quantitatively or semi-quantitatively determined by measuring a signal generated by said complex, said particle is a nanoparticle and said capture molecule is immobilized to said nanoparticle simultaneously with a co-molecule which is smaller than said capture molecule. When the capture molecule is a virus epitope, the co-molecule preferably has a molecular weight in the range of 50-1500 Da, wherein said co-molecules when immobilized on said nanoparticle separate the capture molecules so that an average distance between two adjacent capture molecules is greater than a distance between the antigen binding sites of the anti-virus antibody to be detected.