IL6 and major depressive disorder: Critically, periodontal disease is also associated with high levels of systemic inflammation, in particularly, interleukin-6 (IL-6), TNF-α, and C-reactive protein (CRP; Pussinen et al., 2007; Bansal et al., 2014) that may potentiate inflammatory and oxidative and nitrosative stress processes and thus may lead to a vulnerability to depression (Battino et al., 1999; Chapple and Matthews, 2007; Berk et al., 2013; Bullon et al., 2014).