Innate immunity: Immunity that is naturally present and is not due to prior sensitization to an antigen from, for example, an infection or vaccination.Since it is not stimulated by specific antigens, innate immunity is generally nonspecific.It is in contrast to acquired immunity. Also called natural immunity.nnate immunity: Immunity that is naturally present and is not due to prior sensitization to an antigen from, for example, an infection or vaccination.