CD4 and systemic inflammatory response syndrome: It was found that septic patients, especially those with severe sepsis and septic shock, have significantly higher levels of PD-1 expression in CD4+ or CD8+ T cells, PD-L1 expression in monocytes, sPD-1, and sPD-L1 levels [27,29,31,32,35,36,37], but there was no significant difference between the SIRS (Systemic Inflammatory Response Syndrome) group and healthy controls [34].