CD4 and infection: These gaps prompted us to characterize the clinical symptoms of and the immune response against AIV H9N2 infection in geese, including the viral distribution in tissues during infection, the detection of several immune-related cytokines [IFNα, IFNγ, interleukin (IL)1, IL6, and myeloid differentiation primary response gene 88 (MyD88)], and the distribution of CD4- and CD8α-positive cells.