According to a machine learning study on 54 cytokines, chemokines, and growth factors, bipolar disorder diagnosis was predicted by high levels of inflammatory markers such as C-C motif chemokine ligand 3, interleukin-9 (IL-9), and TNF-α, whereas major depressive disorder diagnosis was predicted by high levels of proinflammatory markers such as IL-1β and IL-6 (Poletti et al., 2021). Here, IL6 is linked to major depressive disorder.