IL-6 is present at low levels in peripheral blood but increases in patients with markers of frailty, during infection, trauma, chronic disease, or other stress.33, 34 It is a pleiotropic cytokine that affects various biological systems and organs.23 IL-6 may increase the activity of indoleamine-2,3-dioxygenase, which is involved in tryptophan metabolism, leading to kynurenine pathway activation and decreased central serotonin availability. Here, IL6 is linked to infection.