On the grounds of a wide scope of cited actual literature, the article describes specificity of the immune system in peri-natal period of life. After presentation of the most basic tasks of the system at the time of transition from intrauterine into the extrauterine environment, the authors describe in multiple details how the new-born’s immune system attains the ability to cope with the new tasks. The main differences in the mechanisms of immune reactivity between the neonatal and adult immune systems are demonstrated. In particular, the development of innate and acquired immune functions are outlined including the role of neutrophils, adhesins, complement cascade, cytokins, numerous antigen presenting cells, different subclasses of T lymphocytes (Th1, Th2, Tregs) and B cells at their stages of naive and engaged cells. The role of passive transfer of humoral immunity from mother is also described. The authors suggest that in contrast to the common opinion on functional disability of neonatal immune system, the system represents the specificities characteristic to the gradual transition from the general tolerant to the active defensive state with preservation of self-tolerance.