The adaptive immune system is switched on by the evolutionarily older innate immune system . This older system is non-specific , whereas the adaptive system is tailored to specific targets . Thought to have arisen in the first jawed vertebrates , the adaptive or `` specific '' immune system is activated by the â non-specific â and evolutionarily older innate immune system ( which is the major system of host defense against pathogens in nearly all other living things ) .