CD40LG and influenza: We describe the different populations of antigen‐experienced IgM cells in relation to their Ig gene repertoire and demonstrate the changes with age at a point 28 days after vaccination with influenza and pneumococcal polysaccharide vaccines.  Additionally, we used a large panel of markers, using mass cytometry, and we show the heterogeneity of IgM memory cells with respect to different levels of IgM expression and identify two distinct populations whose frequencies are altered in aging.