As part of this process, the influx of lymphocytes into the infected lung tissue is commonly observed during experimental infections in vivo; for instance, this process is detectable during influenza infection in mice by 4 dpi (increased levels of CD8+ and CD19+ cells) [38, 39], and in ferrets by 5 dpi (increased mRNA expression of lymphocyte markers) [17]. This evidence concerns the gene CD19 and infection.