Psoriasis is an immune-mediated inflammatory skin disease characterized by the presence of erythematous and itchy plaques. Psoriasis has a multifactorial pathogenesis, environmental and genetic factors contribute to its development. Although interleukin-23 blockade has been shown to be highly effective in the treatment of psoriasis, relapses have occurred during therapy. Our study aims to identify the cellular source of key cytokines involved in disease recurrence or persistence of at least one psoriatic lesion in patients affected by moderate- severe psoriasis treated with an anti-IL-23 biologic drug. The immune infiltrate of resistant or relapsed plaques during anti-IL-23 therapy will be analysed from skin biopsies.