Immune System Practice Test

Natural killer cells provide rapid, non-specific killing of virus-infected or transformed cells. They act without prior exposure or sensitization, unlike cells of the adaptive immune system. NK cells survey for signs of abnormal cells, particularly reduced expression of MHC class I or the presence of stress-induced ligands. When activating cues outweigh inhibitory signals, they unleash cytotoxic granules containing perforin and granzymes to induce apoptosis in the target cell. They can also be boosted by cytokines released during infection, enabling a quick response. This combination of rapid action and lack of prior exposure is what makes NK cells the correct choice for destroying virus-infected or tumor cells without prior encounter. Dendritic cells are primarily antigen-presenting cells that prime adaptive immunity, macrophages are phagocytes, and B cells are adaptive immune cells that produce antibodies.