FGF21 and major depressive disorder: These include proinflammatory cytokines and their receptors (IL-18, IL-18R1, LIF-R), chemokines (IL-8/CXCL8, eotaxin/CCL11, CCL25, CXCL6, MIP-1α/CCL3), biomarkers involved in the cross-talk between innate and adaptive immunity (TNFRSF9, CD5, CD8A) and lastly biomarkers that might be upregulated in depression as part of a counterregulatory response (FGF-21, GDNF).