The biologic treatment armamentarium for rheumatoid arthritis (RA) and psoriatic arthritis (PsA) includes anti-cytokine biologics such as tumor necrosis factor inhibitor (TNFi), interleukin (IL)-1 inhibitor (IL-1i), IL-6 inhibitor (IL-6i), IL-17 inhibitor (IL-17i), IL-12/23 inhibitor (IL-12/23i), and IL-23 inhibitor (IL-23i); T-cell co-stimulation inhibitor (CTLA4-Ig) and B-cell depleting agents are also used. Here, IL17A is linked to rheumatoid arthritis.