BDNF and major depressive disorder: Furthermore, decreased circulating BDNF levels in patients with depression (Molendijk et al., 2014) and positive associations between elevated BDNF and IL-6 levels and between BDNF and leukocyte counts in patients with major depression (Patas et al., 2014) suggest that interactions between T and B cells, cytokines and neurotrophic factors, such as BDNF, may be associated with the neuropathology of depression.