[Ontogeny of the immune system].

The newborn immune system differs quantitatively and functionally from adults. At birth, the immune system is partially immature, resulting in deficiency in cell-mediated cytolysis, immunoglobulin synthesis and cytokine production. The most clearly defined deficit in neonatal phagocytosis defenses is diminished neutrophil storage. T cell function is diminished, including T cell-mediated cytotoxicity and T cell help for B cell differentiation. Selective decreases in cytokine production by T cells may contribute to all of these deficits. One of the fundamental differences between adults and newborns for T cell functions resides in whether or not the patient had prior exposure to antigens. Significant immune responses to antigens can be obtained in the neonatal period. These responses are qualitatively different from those induced in adults with a predominance of TH2 pattern.