Which is a characteristic of natural killer cells?

Natural killer (NK) cells are a crucial component of the innate immune system, known for their ability to directly attack and destroy infected cells and tumor cells without the need for prior sensitization or antibodies. This characteristic distinguishes them from other immune cells that require specific recognition through antibodies or other mechanisms, which is more common in the adaptive immune system.

NK cells recognize stressed cells in the absence of antibodies and enhance the immune response. They are particularly effective against viral infections and certain types of cancer by releasing cytotoxic granules that can induce apoptosis in target cells. This direct action is fundamental to their role in the immune response, helping to eliminate cells that are abnormal or compromised.

In contrast, the other options refer to functions or characteristics not associated with NK cells: they do not require antibodies (which is why the first choice is incorrect), they are not categorized under the adaptive immune system (making the second choice inaccurate), and although they play a role in immune responses, they do not primarily produce immunoglobulins, which are associated with B cells in the adaptive immune system.