Scanning angle reflectometry (SAR) is used to investigate the structure of antigen/antibody layers deposited on a silica surface. The reflectivity curves are analyzed by the means of the optical invariants, which permits the determination of three structural characteristic parameters for the layer. A new invariant is constructed that yields information about the repartition of mass in the layer and thus goes beyond the usual uniform layer model, In our case, this model does not held : the layer appears to be denser away from the adsorbing silica surface than near that surface.