IL2 and infection: Moreover, TNF-α combining with TNFR1 to promote T cell apoptosis is a characteristic of aged T cells (Gupta et al., 2005), IL-2 is an essential cytokine for the proliferation, differentiation, and function of T cells (Ross and Cantrell, 2018), and secretion of cytokines can draw T lymphocytes from blood to the site of infection, which may explain lymphopenia in patients with COVID-19 (Xu et al., 2020).