CD19 and major depressive disorder: According to our knowledge, this is the first study to provide valuable evidence that CD19+ B cell counts, CD3+ T cell counts, CD3–CD16/56+ NK cell counts, and total lymphocyte counts were diagnostic markers for individualized prediction of BD and MDD, which paves the way for the subsequent selection of appropriate clinical treatment strategies (e.g., drug selection) to guarantee a favorable prognosis and avoid unwanted outcomes.