We compared frequencies and numbers of CD4+ memory T cells with specificity for distinct vaccines and infectious pathogens, in blood and bone marrow of the same individuals, by identifying antigen‐reactive T cells ex vivo.33 It turned out that in most adult human donors CD4+ memory T cells specific for viral pathogens encountered in childhood, either by infection or by vaccination, like measles, rubella, and mumps, were maintained exclusively in the bone marrow. Here, CD4 is linked to rubella.