Here, we found that the ratio of CD8+ T cells and the expression of IFN-γ and cytotoxicity-associated genes, including granzyme A/K, perforin and IL2, at 7 days post-infection in peripheral blood mononuclear cells (PBMCs) from B1 haplotype ducks significantly increased in the context of defending against H5N1 AIV infection in vivo. The gene discussed is CD8A; the disease is infection.