Studies have also looked at cell populations of CD4+ and CD8+ T-cells, using influenza-specific IFN-γ responses to characterise leukocyte composition and antigen-specific T-cell responses in key lymphoid tissue following influenza infection in ferrets using an enzyme-linked immunospot (ELISpot) method [21] as well as daily tracking of peripheral blood leukocytes in infected ferrets using a flow cytometric technique [16]. This evidence concerns the gene CD4 and influenza.