We investigated the adsorption and chemical conjugation of avidin and its deglycosylated form, neutravidin, on films of regenerated and nanofibrillar cellulose. The dynamics and extent of biomolecular attachment were monitored in situ by quartz crystal microbalance microgravimetry and ex situ via surface analyses with atomic force microscopy and X-ray photoelectron spectroscopy. The installation of carboxyl groups on cellulose after modification with carboxymethylated cellulose (CMC) or TEMPO-oxidation significantly increases physisorption of avidins, which can be then covalently conjugated by using 1-ethyl-3-[3-dimethylaminopropyl]carbodiimide hydrochloride/N-hydroxysuccinimide (EDS/NHS) coupling chemistries. The developed cellulose-avidin biointerfaces are able to scavenge biotinylated molecules from solution as demonstrated by successful surface complexation of biotinylated bovine serum albumin (Biotin-BSA) and antihuman immunoglobulin G (Biotin-anti-hIgG). Finally, we show that cellulose substrates carrying immobilized anti-hIgG are effective in detecting human immunoglobulin G (hIgG) from fluid matrices.