What is the primary function of natural killer cells in the immune system?

The primary function of natural killer (NK) cells in the immune system is indeed to attack nonselectively on nonself cells. Natural killer cells play a crucial role in the innate immune response, which is the body's first line of defense against infections and tumors. Unlike other immune cells that are typically more specific in their targeting, NK cells can recognize and eliminate infected or cancerous cells without prior sensitization to specific antigens.

This nonselective approach is vital in quickly responding to a wide range of threats, particularly when the adaptive immune system has not yet been activated. NK cells achieve this through mechanisms such as the release of cytotoxic granules that induce apoptosis (programmed cell death) in target cells, as well as producing cytokines that can enhance the overall immune response.

In contrast, the other options reflect functions that are not primarily associated with NK cells. For instance, secreting immunoglobulins is specifically a role of B cells in the adaptive immune response, while heightened selectivity describes a function of T cells, which target specific antigens. Enhancing immune activity through cytokine secretion typically relates to helper T cells and other immune mediators rather than the straightforward attack behavior exhibited by NK cells.