Although IL1β is mainly produced by monocytes in the periphery, which, when activated, infiltrate the brain and interact with microglial cells contributing to the pathophysiology of MDD (30), IL1β protein levels have as yet been investigated in MDD using either plasma or serum assays; mononuclear cell assays have been used in MDD only in transcriptional studies to measure IL1β mRNA (31–34). This evidence concerns the gene IL1B and major depressive disorder.