Semiconducting nanowires have the potential to function as highly sensitive and selective sensors for the label-free detection of low concentrations of pathogenic microorganisms1, 2, 3, 4, 5, 6, 7, 8, 9, 10. Successful solution-phase nanowire sensing has been demonstrated for ions3, small molecules4, proteins5, 6, DNA7 and viruses8; however, 'bottom-up' nanowires (or similarly configured carbon nanotubes11) used for these demonstrations require hybrid fabrication schemes12, 13, which result in severe integration issues that have hindered widespread application. Alternative 'top-down' fabrication methods of nanowire-like devices9, 10, 14, 15, 16, 17 produce disappointing performance because of process-induced material and device degradation. Here we report an approach that uses complementary metal oxide semiconductor (CMOS) field effect transistor compatible technology and hence demonstrate the specific label-free detection of below 100 femtomolar concentrations of antibodies as well as real-time monitoring of the cellular immune response. This approach eliminates the need for hybrid methods and enables system-scale integration of these sensors with signal processing and information systems. Additionally, the ability to monitor antibody binding and sense the cellular immune response in real time with readily available technology should facilitate widespread diagnostic applications.