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A regenerable flow-through sensor based on white light reflectance spectroscopy for label-free detection of proteins and DNA is presented. Detection is based on monitoring the interference wavelength shift caused by an increase in film thickness during bioreactions. By employing an anti-mouse IgG antibody/mouse IgG assay, a detection sensitivity of 150 pM was determined. In addition, the sensor response remained stable after up to 10 regeneration cycles. The sensor was applied for the quantitative determination of complement activation product C3b in human serum, a candidate biomarker for estimating the status of autoimmune diseases. Complement activation products in human serum could be detected at dilutions as high as 6000-fold. Detection of DNA hybridization at concentrations of 5 nM was easily achieved. The proposed sensor is characterized by low cost, short analysis time, high assay sensitivity and reliability.