Sepsis is defined as a life-threatening organ dysfunction that is caused by a dysregulated host response to infection. Severe sepsis is the most common cause of death among critically ill patients in non-coronary intensive care units (ICU). Sustained excessive inflammation and immune dysfunction have been confirmed to play a key role in organ damage and early death of sepsis patients. Therefore, it is important to reduce excessive inflammatory response mediated by immune cells and pro-inflammatory cytokines in the acute phase of sepsis. Single-cell RNA sequencing performed on both septic patients and mice suggest that changes in Tcm (CD3+ CD8+ CD44+ CD127+ CD62L+) and Tem (CD3+ CD8+ CD44+ CD127+ CD62L -) in the acute phase of sepsis may play an important role in sepsis. In addition, animal researches showed that Tcm and Tem decreased decreased continuously at 24, 48 and 72h after cecal ligation and perforation (CLP) in mice, and the adoptive transfer of Tcm , sorting from spleen of mice 24h after CLP , but not Tem improved 7-day survival rate of sepsis mice. This observational study is aimed to investigate the quantity and proliferation of Tcm and Tem in the acute phase of sepsis and their correlation with severity level and mortality of septic patients in ICU.