CD4 and infection: 5-day-old Pekin ducklings were infected with the R38K, T151A, and R304M mutant viruses and the rY and rPS control viruses, and the levels of IL-2, IL-17, IFN-γ, TNF-β, CD4, and CD8 mRNAs in brain and thymus were measured at 7 days after infection (Figure 7B).