Many types of infiltrating immune cells, including macrophages, T cells, mast cells, and B cells, as well as their cytokines, including IL-1β, TNFα, and IL-6, are present at higher levels in the OA synovium than in the normal synovium [28], although the overall number of immune cells in the OA synovium is lower than that in the rheumatoid arthritis (RA) synovium [34]. Here, TNF is linked to rheumatoid arthritis.