CTSB and mastitis: Proteins involved in innate immunity and antimicrobial functions (e.g., serotransferrin, complement C3, fibrinogen gamma-B chain, and cathepsin B) and are associated with the immune response to pathogens (e.g., polymeric immunoglobulin receptor-like protein, MHC class I antigen, and beta-2-microglobulin) are abundantly expressed in whey from S. aureus mastitis milk27.