When COVID-19 induces viral infections, the B cells induced by follicular helper CD4+ T cells could produce antibodies capable of neutralizing the pathogen and can combine with cytolytic CD8+ T cells to kill pathogen-infected cells, which leads to a decrease in peripheral blood lymphocytes, while the COVID-19 vaccine could induce robust lymphocyte generation, which led to elevated peripheral blood lymphocytes. The gene discussed is CD4; the disease is COVID-19.