IL17A and ankylosing spondylitis: Ankylosing spondylitis shares more similarities to psoriatic arthritis and psoriasis than rheumatoid arthritis, since ankylosing spondylitis shows serological negativity for rheumatoid factor, and an inflammatory cytokine pattern based on increased production of tumor necrosis factor alpha (TNF-α) and interleukin (IL)-17A [9].