The extracellular expression of C5aR1 on NK cells from preterm infants without infection (median-MFI = 3263) was significantly higher compared to infants with EOS or born to mothers with AIS (median-MFI = 2299) (Mann–Whitney test p = 0.009, Figure 4), especially on the CD56dim CD16- subset. Here, C5AR1 is linked to infection.