IL17A and psoriasis: Many previous studies revealed that psoriasis is an inflammatory disease, and patients with this disease may have higher serum levels of cytokines, such as tumour necrosis factor (TNF)-α, interferon (IFN)-γ, (interleukin (IL)-6, IL-8, IL-12, IL-17 and IL-18 (Arican et al., 2005; Lowes et al., 2014; Baliwag et al., 2015).