Animal studies have revealed the critical molecular insights of CD4+ cells in SARS-CoV clearance and attenuation of a pathological condition. The depletion of CD4+ cells was associated with reduced virus clearance and interstitial pneumonitis (Jin et al., 2005; Wang et al., 2006). In comparison, the adoptive transfer of virus-specific CD4+ and CD8+ T cells resulted in viral clearance (Zhao et al., 2010). Similarly, clinical data has consistently shown the presence of antigen-specific CD4+ and CD8+ T cells in the recovered patients, akin to what was found in immunized animals, reviewed in Channappanavar et al. (2014). On the other hand, severe cases of SARS-CoV infection were associated with a decline in T cells, as will be discussed in later sections. Thus, based on these animal and clinical data, CD4+ T and CD8+ T cells were central to the antiviral response during SARS-CoV infection (Peng et al., 2006; Oh et al., 2012).