CD4 and infection: To further investigate the effects of age on immune responses to DTMUV infection, we evaluated the dynamics of T helper lymphocyte (Th, CD3+CD4+), cytotoxic T lymphocyte (CTL, CD3+CD8+), B lymphocyte (CD3-IgY+), and non-T and B (CD3- IgY-) lymphocyte responses, as well as neutralizing antibody responses, following DTMUV infection in adult laying ducks (27 weeks old).