Although cytokines are broadly divided into pro-inflammatory (e.g., IL-1β, IL-6, TNF-α) and anti-inflammatory (e.g., interleukin 4—IL-4, interleukin 10—IL-10, transforming growth factor β1—TGF-β1), some, including IL-6 and TGF-β1, display context-dependent activity, which may contribute to the complex and heterogeneous immune profile observed in major depressive disorder. Here, IL4 is linked to major depressive disorder.