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As part of clinical diagnosis, a clinician is required to detect disease causing antigens, bacteria, or viruses in serum, saliva, or other biological samples. Usually, this requires the sample to be sent to a pathology laboratory for analysis. Silicon nanowires can be made into sensitive molecular sensors. When being functionalized with antibodies, they are capable of detecting femto molar concentrations of antigens in real time. Biological molecules at a pH different from their isoelectric point exhibit a net charge. When an antigen attaches to the antibody on the nanowire, the net charged on the antigen displaces free carriers in the nanowire changing its conductance. To date, detection methods have been based upon directly measuring the change in dc conductance. This is difficult and requires sensitive low-noise amplifiers and high-resolution analog-to-digital converters. This is not ideal for low-cost and highly integrated systems. In this paper, it is demonstrated that nanowires exhibit an ac-transfer function that resembles that of a high-pass filter. To the authors' knowledge, this is the first time this effect has been reported. Furthermore, it is illustrated that as molecules with a higher net charge attach to the nanowire and displace more charge carriers within the nanowire channel, the filter's corner frequency decreases. This property of silicon nanowires is exploited to build a low-cost real-time antigen detection system.