Numerous studies showed that PRRSV infection or attenuated modified live virus (MLV) vaccination induced a relatively weak and delayed cellular immune response, where increased cytotoxic CD8 T cells and CD4+CD8+ T cells were detected at 4–5 weeks after infection or immunization [18,19], while IFN-γ-producing cells were occasionally detected as early as 2–3 weeks in individual animals after infection [20,21]. Here, CD4 is linked to infection.