In our study, an increase in the CD4+/CD8+ T cell ratio, M%, and albumin level was observed, single variables (CD4+/CD8+ T cell ratio, M%, and albumin level) and two-variable combinations (CD4+/CD8+ T cell ratio and M%, CD4+/CD8+ T cell ratio and albumin level, and albumin level and M% exhibited a low AUC for predicting the severity of depression in comparison with the three-variable combination (joint index). Here, CD4 is linked to major depressive disorder.