Spearman correlation analysis indicated that other clinical variables, including onset age of MDD, duration of illness, HAMD-24 score and HAMA score, were not associated with the levels of C3, C3a, C1q and CRP in the MDD group (Table S1), while after controlling for BMI and age levels of C3 was found to be associated with HAMA score (Table S1). Here, C3 is linked to major depressive disorder.