TNF and psoriatic arthritis: Proteins in the immune system, such as tumor necrosis factor-α (TNF-α), interleukins (ILs) 12 and 23, or interleukin (IL) 17 play a crucial role in the pathogenesis of many diseases such as psoriasis, psoriatic arthritis, rheumatoid arthritis, or ankylosing spondylitis.