TNF and rheumatoid arthritis: SpA is one of the more common extra-intestinal manifestations in the setting of IBD, and a shared pathogenesis underlying these diseases is emerging, including genetic predisposition, environmental factors, and pro-inflammatory cytokine pathways (i.e., tumor necrosis factor [TNF], interleukin [IL]-23, and IL-17).[7,8,9,10] Conversely, the relationship between IBD and RA is less clear.[11,12]