CD4 and bacterial infectious disease: As shown in Fig. 5c, immunization with SyBV induced a rapid increase in proliferating CD4+ T-cells in the spleen, including an increase in follicular B-helper T-cells (Fig. 5d) and memory T-cells (CD4+CD44+) (Fig. 5e), showing that bacterial SyBV immunization stimulates an extensive B-cell and T-cell response to protect against bacterial infection.