Abstract: The present invention relates to a solid-phase immunoassay system for the determination of an antibody or an antigen in a sample which consists of an interferometric signal from an optical source, a solid support coated with an antibody or an antigen and having at least one region immersed in a solution containing a sample, thereby the corresponding antigen or antibody can be complexed on the solid support, an optical transfer and focusing system to measure the interferometric signal after its transmission (or reflectance) by the solid support, and a measuring device to record and process the interferometric signal at a wavelength corresponding to an absorption characteristic of the antigen-antibody complex or of a label incorporated into the antigen-antibody complex, thereby determining the amount of antigen or antibody in the sample.