Figure 7: Principles of protein chip operation. The two distinct modes-of-action rely on either the direct (Forward) or indirect (Reverse) immobilisation of the protein analytes, such as products, product variants and contaminants, to the glass microscope slide by their non-covalent and chemical crosslinking or binding to immobilised antibodies, respectively. The protein analytes are subsequently detected by labeled antibodies in the “sandwich” or by labeling of the analytes in the “labeling” configuration. Read-out is identical for all modes and configurations and compatible with the micro- and nanoarray formats.