CD4 and influenza: Assays were carried out with whole (inactivated) H1N1 virus using mixed PBMC (containing detectable influenza-specific IL-2+CD4+ T cells; Supplemental Fig. 3) and pooled human AB serum containing anti-H1N1 Abs to standardize the level of influenza-specific IgG in cultures (see Materials and Methods) (Fig. 3).