C3 and major depressive disorder: Notably, there were 18 common genes differentially expressed between C versus M and A versus M, and these genes are mainly involved in neurodevelopment, inflammation, signal transduction, and cellular communication; in addition, genes including Alas2 [34], C3 [35], Ccl2 [36–38], and Ptx3 [39] have been reported to be associated with depression; the others were rarely reported.