Final answer:
Natural killer (NK) cells secrete interferon-gamma (IFN-γ) to activate macrophages, thus enhancing their response to infections or tumor cells.
Step-by-step explanation:
The cytokine that an NK cell secretes to activate macrophages is called interferon-gamma (IFN-γ). NK cells, or natural killer cells, are a crucial part of the innate immune system and are always active, constantly patrolling the body for signs of infections or tumor cells. When NK cells encounter such cells, they release cytotoxic granules that contain perforin and granzymes, leading to the destruction of the targeted cell.
Besides this direct cytotoxic activity, NK cells also have the ability to regulate the immune response. The release of IFN-γ by NK cells has an activating effect on macrophages, enhancing their phagocytic ability and encouraging the inflammatory response to fight off infections or tumor cells.