On one side, mammalian cathepsins S, L, and V represent key players in the vertebrate immunity through their participation in the antigen presentation processes of DCs and macrophages, on the other hand, they are involved in many pathological processes, such as psoriasis, muscular dystrophy, neurodegenerative disorders, cancer, etc. Sialostatin L strongly decreases the production of IL-9 by Th9 cells, essentially involved in the induction of asthma symptoms. Here, IL9 is linked to asthma.