Final answer:
Natural killer (NK) cells can kill infected or cancerous cells, spare normal cells, and accomplish extracellular killing. They are not part of the third line of defense in the immune system.
Step-by-step explanation:
Natural killer (NK) cells are a type of lymphocyte that can kill cells infected with viruses or tumor cells. They are involved in the removal of neoplastic cells (cancer cells) and identify and spare normal cells. NK cells accomplish extracellular killing, meaning they can induce apoptosis (programmed cell death) in target cells without directly contacting them.
However, the false statement among the given options is E) they are part of the third line of defense. Natural killer cells are part of the innate immune response, which is the first line of defense, not the third line.