Using complementary approaches of vitamin A deficiency and mice for which response to the vitamin A metabolite RA, was abrogated in T cells, we identified a previously unrecognized role for retinoic acid and its receptor, retinoic acid receptor alpha, in the regulation of early T cell activation events and T cell polarization. Impaired response of T cells to the vitamin A metabolite RA was associated with defect in protective immunity to mucosal infection and altered response to oral vaccine. Our work also demonstrates that the vitamin A metabolite, RA, plays a role in the control of dendritic cells function and development. Notably, we found that RA regulates pre-cDC differentiation and that vitamin A deficiency has major consequences on the development of mucosal dendritic cells.