CD4 and susceptibility to pneumonia measurement: Preoperative samples from patients who ultimately developed a postoperative pneumonia demonstrated reduced numbers of naïve CD4 T cells and B cells, and significantly elevated numbers of mature neutrophils, hematopoietic stem and progenitor cells (HSPCs), and S100A8/9-high neutrophils (Fig. 3B; Supplementary Table 5, Supplemental Digital Content 6, http://links.lww.com/SLA/E791).