CCL2 and major depressive disorder: 2019). C‐reactive protein (CRP) and IL‐6 were elevated in MDD, SZ, and PTSD, while CCL2 was particularly increased in MDD (Yuan et al. 2019). Notably, IL‐1β distinguishes MDD from BD, being elevated in the former but reduced in the latter (Yuan et al. 2019). Mendelian randomization studies support a causal role for IL‐6 in MDD and SZ, further affirming the mechanistic involvement of chronic low‐grade inflammation in disease onset and progression (Perry et al. 2021, Zhang et al. 2023).