However, an earlier immune response in lung-draining lymph nodes—as measured by total cellularity and CD44+ CD62L− CD4+ T cell numbers— was apparent at 14 days after infection in C57BL/6 than in C3HeB/FeJ mice, with similar responses detectable in both mouse strains by day 20 (Fig. S1, h and i). This evidence concerns the gene CD4 and infection.