ICOS and influenza: Both CXCR5- and CXCR5+ subsets expanded seven days after influenza vaccination, while at day 28, most CD4+IL-21+ICOS+ TH cells were CXCR5-, suggesting that antigen-specific memory TFH cells might be in a resting state in the periphery and transiently express CXCR5 homing receptor upon antigen encounter.