Cell-mediated immunity is a crucial part of the immune system that primarily involves T cells, a type of white blood cell. These cells recognize and respond to infected or abnormal cells, such as those infected by viruses or cancerous cells. When activated, T cells can directly kill these harmful cells or help coordinate the immune response by signaling other immune cells. This process is essential for defending the body against intracellular pathogens and plays a significant role in transplant rejection. Cell-mediated immunity is distinct from humoral immunity, which involves B cells and the production of antibodies. Together, they form a comprehensive defense system.