Final answer:
Natural killer cells are the innate immune system cells that attack host cells containing an intracellular pathogen, functioning without prior exposure to the pathogen.
Step-by-step explanation:
The innate immune system cells that attack host cells harboring an intracellular pathogen are natural killer cells (NK cells). These cells are a vital part of the body's first line of defense and are capable of attacking cells that have lost normal cell surface markers due to viral infection or cancerous transformation. Unlike cytotoxic T-cells, which are part of the adaptive immune system, NK cells do not require previous exposure to a pathogen to become activated.