CD4 and influenza: In line with the association between an “early” activation phenotype and strong IgG induction, the percentage of HA118-132-specific ICOS+CD38++ CD4 T cells at day 4 as well as the percentage of HA118-132-specific cTfh cells at day 4 correlated with the FC of HA-specific antibodies (Fig. 5d and Supplementary Fig. 6d), suggesting a direct link between early influenza-specific CD4 T cell activation and antibody induction.