In this study, higher S100A9 but lower CD163 expression was found in IgG4-RD MNs and stronger signals of MNs and profibrotic macrophages (M2) in IgG4-RD tissues compared with HCs (Figure 5F and Supplemental Figure 5E). Here, S100A9 is linked to immunoglobulin G4-related sclerosing disease.