Natural Killer (NK) cells were previously thought to be a part of the innate immune response. Mounting evidence of memory in NK cells suggests that these cells may possess characteristics previously thought to be in the domain of the adaptive immune system (see figure Innate and adaptive immunity).he innate and adaptive immune responses are distinct arms of the immune system that can be categorized based on the ability to learn from a primary exposure. Upon secondary exposure to a pathogen, the cells of the adaptive immune response will exert their effector function in a more rapid, robust way.