LEP and juvenile idiopathic arthritis: As a part of the study, an assessment was also conducted to evaluate the correlation between the examined biological markers—tenascin C and adiponectin and leptin concentrations—in the blood of children with juvenile idiopathic arthritis before the initiation of etanercept therapy, as well as at various time points during treatment, specifically at 3, 6, 12, 18, and 24 months of therapy.