Studies have shown that the following pro-inflammatory markers have been found to be at increased levels in depressed individuals: CRP (79, 80), IL-6 (22, 79, 81, 82), TNF–α, and interleukin-1 receptor antagonist (IL-1ra) (79, 82), however, this association is not unidirectional and the subsequent development of depression also increases pro-inflammatory markers (82, 83). Here, IL6 is linked to major depressive disorder.