CD4 and major depressive disorder: A comprehensive meta-analysis of 27 studies reported increases in absolute counts of white blood cells: granulocytes, neutrophils, monocytes, CD4+ T cells, natural killer (NK) cells, CD19+ B cells, and activated T cells (particularly CD25+ and HLA-DR+ subsets) in MDD patients compared with healthy controls, with relative reductions in helper T (Th)1 and Th2 cells but largely unchanged numbers of other T cell subsets, including CD3+, CD8+, naive and memory T cells and regulatory T (Treg) cells (Foley et al., 2023).