Recently, the novel coronavirus disease (COVID-19) has broken out worldwide, 1 with rapid increase of infected patients.COVID-19 dominantly leads to pneumonia. 2Among these COVID-19 patients, some appears to be severe symptoms with acute respiratory distress syndrome, organ failure, 2 and further present a poor outcome.Previous studies have been reported that immune patterns are closely associated with disease progression of patients infected with other viruses. 3The correlation between immune signatures and outcome of severe and critical COVID-19 cases was not well illuminated.Therefore, we aimed to evaluate the correlation between immune characteristics, especially levels of lymphocytes and cytokines in peripheral blood, and clinical parameters in severe and critical COVID-19 patients, in order to find critical indicators of disease progression and to provide important guides for therapeutic strategy.Thirty-six adult cases with severe and critical COVID-19 were enrolled.The disease outcome, immune patterns, microbiota infection, coagulation profile, and organ dysfunction biomarkers were analyzed and collected.This project was approved by the Ethics Committee of our hospital (No. 2020-KY-060), and all patients signed the informed consent.We found that the cell numbers of lymphocytes in these patients were obviously decreased compared to that in healthy donors, including total lymphocytes, total T, CD4 + T, CD8 + T, B and NK cells (Fig. 1a), and the percentage of lymphocytes in COVID-19 patients was also significantly decreased except B cells (Supplementary Fig. S1a,b), with an increased ratio of CD4 + /CD8 + T cells (Supplementary Fig. S1c), suggesting that CD8 + T cell number was more decreased than CD4 + T cell number in these patients.Particularly and importantly, the mean value of only B cell number (150.05/μL) was within normal range [(90-560)/μL] (Fig. 1a).All these data indicate that lymphocytes are impaired in severe and critical COVID-19 patients, especially more impaired in CD8 + T cells than in CD4 + T cells, and little impaired in B cells, which results in the higher percentage of B cells.Furthermore, the cytokine levels, including IL-2, IL-4, IL-6, IL-10, TNFα, and IFN-γ, were remarkably increased; especially IL-6 and IL-10 (Fig. 1a, Supplementary Fig. S2a).The results presented that the maximum values of IL-2, IL-4, IL-6, IL-10, TNF-α, and IFN-γ were 16.66, 16.87, >14031.14,7451.48,155.53, and 453.95 (pg/mL), respectively (Supplementary Fig. S2b).Moreover, the percentages of patients with IL-6 and IL-10 upregulation were 97.0% and 100.0%, respectively, which were significantly higher than that of patients with other cytokine upregulation (Supplementary Fig. S2b).In addition, the levels of IL-6 and IL-10 in critical COVID-19 patients were significantly higher than that in severe COVID-19 patients (Supplementary Fig. S2c).These findings demonstrate that cytokine level was elevated in severe and critical COVID-19 patients, particularly IL-6 and IL-10 were enormously increased.To further evaluate the correlation between these immune parameters and clinical prognosis, we analyzed the overall survivals