CD4 and COVID-19: Consistently, another study found that the levels of tumor necrosis factor-α (TNF-α) and IFN-γ in CD4+ T cells of severe COVID-19 patients were lower than those of mild patients, while the levels of granzyme B and perforin in CD8+ T cells of severe patients were higher than those of mild patients (Zheng H. Y. et al., 2020).