TNF and depressive disorder: Individuals with MDD also show elevated serum levels of various immune factors such as IL-6 (Engler et al., 2017; Ye et al., 2018) and TNFα (Fan et al., 2017; Zou et al., 2018), and IL-6 in particular has been implicated in depression across a number of clinical and preclinical studies (for review, see Hodes et al., 2016).