CD4 and major depressive disorder: In the comparison of peripheral blood laboratory parameters, the levels of most immune cells, including CD3+ T cell%, CD3 + CD4+ T cell%, CD4+/CD8+ T cell ratio, CD19+ B cell%, CD16 + 56+ NK cell%, and the CD16 + 56+ NK cell count of the healthy population were higher than that of MDD patients in the training cohort.