Newborn babies have immature immune systems and an increased risk of infections, especially those born prematurely. Very preterm infants are also at risk of other complications such as necrotizing colitis (NEC), retinopathy (ROP) and chronic pulmonary disease (BPD), and all of these dreaded complications are inflammatory conditions involving the infant's immune system. At present, we do not fully understand the factors that give rise to these inflammatory conditions, nor can we predict with certainty which preterm infants will be affected. In this project, we conduct the most detailed mapping of the neonatal immune system to date in order to better understand the immunological processes associated with inflammation (NEC, ROP and BPD) and severe infections in preterm infants. Our goal is both to understand these processes better, but also to identify markers that can help us predict the risk of infection in a given preterm infant. In this way, we hope to optimize the care of preterm infants in the future with reduced risk of complications and increased survival in this vulnerable patient group.