They investigated C-reactive protein (CRP), interleukin 6 (IL-6), interleukin 10 (IL-10), and various leukocyte populations (lymphocytes, neutrophils, monocytes, T-helper and cytotoxic T-cells, B cells, and natural killer (NK) cells; Euteneuer et al., 2017) by obtaining plasma from MDD patients and controls. Here, CRP is linked to major depressive disorder.