In severe COVID-19 patients [Diagnostic criteria: meeting one of three criteria: (1) dyspnea, RR>30 times/min, (2) oxygen saturation <93% in ambient air, (3) PaO2/FiO2<300 mmHg (Wang Y. et al., 2020)], the number of CD4+ T cells, CD8+ T cells, B cells and natural killer (NK) cells, as well as the percentage of monocytes, eosinophils and basophils were reduced significantly (Huang et al., 2020; Qin et al., 2020; Xu et al., 2020). The gene discussed is CD4; the disease is COVID-19.