CD4 and infection: Proliferating Ki67+ CD4+ T cells can be detected in the tracheobronchial lymph nodes (TBLN) of experimentally infected pigs as early as 4 days after infection (Talker et al., 2016), and IFN-γ secreting CD8+ T cells can migrate to the lungs and bronchioalveolar lavage (BAL) by 6 days after infection (Khatri et al., 2010; Talker et al., 2016).