CD4 and infection: 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.