**2. The immune system**

The immune system is a complex network designed to react against harmful foreign agents as well as pathogens. The immune system is immature during fetal and neonatal life. The fetus receives passive protection from the mother *in utero*, thanks to maternal immunoglobulins (Ig)G, which can cross the placental barrier. This protection continues until the first months of the life of the newborn, as maternal IgG are transferred also through breastfeeding. The development of the immune system begins early in life and is greatly influenced by the type of feeding and environmental exposure (including factors such as the presence of domestic animals, antibiotic use, and timing of introduction of different foods from weaning). In the elderly, a progressive decline of the immune function is observed, a process known as immunosenescence [5]. Two types of immune response exist: the nonspecific or innate response, which is the first line of defense and operates in a nonselective way against foreign antigens, and the specific or adaptive one, which is triggered after exposure to a particular antigen. Both responses have a cellular and a humoral component. The two responses are interconnected for several reasons: 1−The cytokines (soluble mediator molecules) secreted in the early stages of the innate response influence the type of adaptive immune response that will develop; 2−Macrophages and dendritic cells, activated during the innate response, act as antigen-presenting cells (APC) for naive (i.e., having not yet encountered the antigen) T lymphocytes, inducing their differentiation into effector T lymphocytes; 3 - In some cases, the phagocytosis performed by innate immune cells is more efficient, if the microorganism to be cleared has been previously bound and surrounded by antibodies (opsonization). The nature of the antigen determines which of the two responses is preferentially activated; however, a complete immune response requires the coordinated participation of both types, and it ends once the trigger is resolved (self-limiting capacity) [6].
