The objective of this study is 1/ to determine the rate and kinetics of MAIT cell expansion and maturation in neonates in relation with gestational age, and in HSCT recipient children in relation with the source of donor stem cells, 2/ to correlate gut microbiota diversity and function with MAIT cell maturation and function in neonates and HSCT recipients; and 3/ to link MAIT cells and gut microbiota composition with microbial infections and severe intestinal inflammatory events in term and preterm neonates, and in HSCT recipients