Human NK cells express C5aR1 extra- and intracellularly; however, intracellular C5aR1 further increases in preterm infants (213, 278). In mice, naïve NK and NKT cells do not express active extracellular C5aR1, which is seen only during inflammatory conditions, like sepsis (279, 280). Here, C5AR1 is linked to Sepsis.