Cell-mediated immunity is a crucial part of the immune system that involves the activation of specific white blood cells called T cells. These cells recognize and respond to infected or abnormal cells, such as those infected by viruses or cancerous cells. When T cells encounter these threats, they become activated and can directly kill the infected cells or help coordinate the immune response. This type of immunity is different from humoral immunity, which relies on antibodies produced by B cells. Cell-mediated immunity is essential for controlling infections that do not respond well to antibodies, making it vital for overall health and protection against various diseases.