SAW biosensor

Hi,

I am newbie into the biosensor field. I am planning to work on the SAW biosensor. I have a very basic doubt regarding the mass loading principle employed in SAW. In the case of immunosensors with SAW, binding of antigen to antibody gives rise to a mass change in the crystal and this leads to a resonant frequency drift. Now this brings in me a confusion, any element be it the respective antigen or not, could lead to a change in the mass. So how can one predict that the resonant frequency change due to mass loading corresponds to the target analyte present in the analyte?