Then the patients with MDD, stratified by suicidal ideation severity were assessed for peripheral cytokine levels (interleukin [IL])-2, IL-4, IL-6, IL-10, interferon-gamma [IFN-γ], IL-17, IL-12, IL-27, and tumor necrosis factor-α) using flow cytometry and enzyme-linked immunosorbent assay. Here, IL6 is linked to major depressive disorder.