What role do natural killer (NK) cells play in the immune response?

Natural killer (NK) cells are a vital component of the innate immune system, and their primary role is to identify and eliminate virus-infected cells and tumor cells without the need for prior sensitization. This spontaneous ability to target impaired cells allows NK cells to act rapidly in response to infections or abnormal cell growth, and they do this by recognizing stressed cells in the body. NK cells can discern infected or transformed cells through various surface markers and trigger apoptosis (programmed cell death) in those cells.

Unlike other immune cells that may require previous exposure to a specific pathogen (like T cells), NK cells are part of the body's first line of defense and can respond immediately, making them crucial for early immune responses. They do not produce antibodies; that function is attributed to B cells in the adaptive immune response. NK cells also do not have a direct role in activating other immune cells, although their activity can influence the overall immune response. Lastly, they do not facilitate blood clotting, which is a function associated with platelets and certain plasma factors in hemostasis. Thus, the defining characteristic of NK cells is their ability to destroy infected or malignant cells swiftly and efficiently without prior exposure.