Described herein is a modification unit (37) for the continuous, pathogen reduced processing of a guidance molecule e.g. a conjugate of a peptide or a protein or a nucleic acid and a linker comprising the following components: at least one reservoir containing the guidance molecule in buffer solution (1) and/or at least one inlet for a product stream containing the guidance molecule in buffer solution, at least one reservoir containing the linker in solution (2), at least one mixing device (3), at least two valves (7, 8), one for dosage of the guidance molecule and one for dosage of the linker molecule at least one outlet for the product stream comprising the guidance molecule-linker complexes and/or a reservoir for taking up the guidance molecule-linker complexes (5) further comprising at least one residence time device ensuring a defined residence time, i.e. ensuring that after mixing the guidance molecules and the linker molecules always spend a similar amount of time in a continuous process.