Little research has been carried out in the influence of TLR9 over the serotonergic system, and only a few works have indicated that SSRIs may modulate TLR9 mRNA expression in the peripheral blood mononuclear cells of depression patients (181) and will be implicated in the tryptophan catabolism (i.e., the main 5-HT resource) (193). This evidence concerns the gene TLR9 and major depressive disorder.