Complementing these findings, systematic reviews and meta-analyses of cerebrospinal fluid biomarkers report higher CSF IL-6 and other inflammatory changes in unipolar depression, alongside broader alterations in neuroendocrine and neuroplasticity-related proteins consistent with central immune activation and blood–brain barrier involvement (Enache et al., 2019; Mousten et al., 2022). Here, IL6 is linked to major depressive disorder.