PDCD1 and susceptibility to pneumonia measurement: PD-1/PD-L1 signaling has been shown to be pivotal in regulating immune dysfunction, particularly in conditions such as sepsis (10), the soluble forms of these molecules, sPD-1 and sPD-L1, have emerged as rapid immune biomarkers and have demonstrated utility in the diagnosis and risk stratification of various critical illnesses similar to severe pneumonia (SP) (11–14).