Final answer:
Natural killer cells are activated by TH1 cells, which secrete cytokines that enhance the activity of various immune cells, including natural killer cells themselves.
Step-by-step explanation:
Natural killer cells are a crucial part of the innate immune system, playing a role in the defense against tumors and virally infected cells. These cells are activated by various mechanisms, including the influence of other immune cells such as T helper cells. TH1 cells are a subtype of helper T lymphocytes which secrete cytokines that not only stimulate and enhance the activity of macrophages and cytotoxic T cells but also activate natural killer cells, improving their ability to destroy target cells. This is particularly important for targeting cells that are harboring intracellular pathogens or are otherwise abnormal, as the TH1 response is directed towards intracellular invaders.