TNF and psoriatic arthritis: TNF-α is involved in the initiation and regulation of the inflammatory response, by inducing inflammatory cytokines such as IL-1 and IL-6, by recruiting immune and inflammatory cells and by inducing the expression of adhesion molecules as suggested by high levels of TNF-α observed in patients with ankylosing spondylitis (AS) and psoriatic arthritis (PsA) [57,58].