TNF and major depressive disorder: Likewise, heightened levels of IL-1β, IL-6, IL-12, IL-18, TGF-β, and TNF-α are linked to AD (Swardfager et al., 2010), and importantly, patients diagnosed with both AD and depression demonstrate the highest levels of IL-6 and TNF-α in their circulatory system (Santos et al., 2016).