Within the RA joint IgG complexes, both soluble and embedded within synovial tissue, activate further ROS production by neutrophils via activation of FcγR2a and FcγR3b (Figure 2) (94, 95), and trigger degranulation of proteolytic enzymes including elastase and cathepsin G (8, 26, 56, 96–98). Here, CTSG is linked to rheumatoid arthritis.