CD4 and infection: Third, the proportions of the CD4 T cell response after IAV infection belonging to Th1, Tfh, or Treg subsets is distinct in the two mouse strains: in C57Bl/6 mice, the Th1 and Tfh subsets represent stable proportions of the total CD4 T cell pool throughout infection, but only the Th1 and Treg contingent is stable throughout infection in BALB/c mice.