Various immune cytokines released during systemic, “low grade” and self-sustaining inflammation have been found to be implicated in the pathophysiology of depression, including interleukins (IL)-1, IL-2, IL-4, IL-6, IL-8 and IL-10; interferon-gamma (IFN-γ); C-reactive protein (CRP); tumour necrosis factor-alpha (TNF-α); and monocyte chemoattractant protein-1 (MCP-1) [23,24]. This evidence concerns the gene IL4 and major depressive disorder.