Elevated serum levels of C-reactive protein (CRP) and the biologically linked interleukin-6 (IL-6), as seen in low-grade inflammation, are among the many proposed causes, and observational studies have reported higher serum CRP levels among people with depression (Gimeno et al., 2009; Howren, Lamkin, & Suls, 2009; Miller & Raison, 2016; Osimo, Baxter, Lewis, Jones, & Khandaker, 2019) and anxiety (Michopoulos, Powers, Gillespie, Ressler, & Jovanovic, 2017; Pitsavos et al., 2006; Vogelzangs, Beekman, de Jonge, & Penninx, 2013). Here, CRP is linked to depressive disorder.