The relative deficiency of T helper type 1 (Th1) and cytotoxic T lymphocyte (CTL) responses in early life is associated with an increased susceptibility to infections by intracellular microorganisms. This is likely to ref lect Newborns and young infants are at enhanced risk of severe infection by intracellular microorganisms such as viruses or certain bacteria for which clearance requires the induction of strong cellular immune responses. The immaturity of CD8 cytotoxic T cells, natural killer (NK) cells, and macrophages in early life has long been recognized. However, it was only recently observed that this impairment of cellular responses could derive from a preferential polarization of immature CD4

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