Cytotoxic cells, also known as cytotoxic T lymphocytes (CTLs), are a type of immune cell that play a crucial role in the body's defense against infections and cancer. They identify and destroy cells that are infected with viruses or have become cancerous. By recognizing specific markers on these unhealthy cells, cytotoxic cells can effectively eliminate them, helping to maintain overall health. These cells are part of the adaptive immune system, which means they can remember past infections and respond more quickly if the same threat reappears. This memory function is essential for long-term immunity and is the basis for how vaccines work, training the immune system to recognize and combat specific pathogens.