CTSB and COVID-19: On the one hand, they can be used by viruses to enter the cell (an increased level of cathepsin B promotes entry, including SARS-CoV-2 infection), on the other hand, they help fight viruses - they are associated with the presentation of antigens (a higher level of cathepsin H increases presentation, as also showed in COVID-19) or help limit viral replication (increased level of cathepsin C) (40, 41).