Fourth, in free-living conventional animals, memory CD4 T cells and B cells accumulate concurrently when driven by exposure to influenza and other infections early in life and drive naïve T and B cells to become Ag-specific long-lived memory cells that maintain their function and often persist throughout life (Haynes et al., 2003; Guerrettaz et al., 2008; Kogut et al., 2012; Devarajan and Swain, 2019). The gene discussed is CD4; the disease is infection.