**5.1 Intestinal immaturity, dysbiosis, and barrier dysfunction**

Immaturity of neonatal intestinal mucosal barrier and mucosal immune system is characterized by decreased mucus coat, altered mucus protein, reduced Ig A, and abnormal epithelial membrane and tight junctions [4, 26, 27]. Preterm intestinal

mucosa is highly immunoreactive, and fetal human enterocytes have been shown to evoke excessive immunological and inflammatory response compared to adults. An imbalance between epithelial cell injury and repair leads to a gut barrier failure and a consequent cycle of bacterial invasion, immune activation, uncontrolled inflammation, and gut necrosis (**Figure 2**).
