IL6 and major depressive disorder: Considering that chronic neuroinflammation is a known biological risk factor for depression [3,4,29], the effect of irisin on the expression of selected pro- and anti-inflammatory cytokines (Interleukin-6 (IL-6), Interleukin-1β (IL-1β), Interleukin-10 (IL-10), Interleukin-4 (IL-4), Interleukin-1 receptor antagonist (IL-1ra)) was evaluated.