TNF and major depressive disorder: In summary, the odds of MDD increased with increasing levels of IL-6; participants with low levels of TNF-α were at reduced risk of MDD compared to participants with no TNF-α, but as the level of TNF-α increased, the risk of MDD increased, and in particular participants with high levels of TNF-α (of 500 or above) were at a significantly increased risk of MDD.