BDNF and major depressive disorder: Several studies have explored the biomarkers for depression via peripheral blood-based proteins and have mainly focused on five systems, involving inflammation (IL-6, CRP), neurotransmitter components (serotonin 1A receptor), neuroendocrine (cortisol), neurotrophic (brain-derived neurotrophic factor, BDNF) and metabolic processes (Kennis et al., 2020; Malik et al., 2021).