Figure 1. Detection of antigen-specific CD4 T cells against influenza viruses by flow cytometry after in vitro expansion of effector cells. Peripheral blood mononuclear cells were expanded in vitro with interleukin-2 (IL-2) for 9 days in the presence or absence of specific influenza antigens, as indicated, then analyzed by flow cytometry by using the intracellular staining assay. The effector T-cell response was analyzed for interferon-gamma (IFN-γ) or IL-2 cytokine expression. Unstimulated cultures (A), CD4 T-cell response against human influenza vaccine strain preparation (B), inactivated avian influenza (H5N1) (C), and H5/N1 peptides (D) are shown in a representative donor.