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Cytotoxic T lymphocytes, often abbreviated as CTLs or cytotoxic T cells, are a critical component of the adaptive immune system. They are a type of T lymphocyte, a white blood cell that plays a central role in cell-mediated immunity. Cytotoxic T cells are primarily responsible for identifying and eliminating infected cells, such as those harboring viruses or intracellular bacteria, as well as cancerous cells.

These cells recognize foreign or abnormal antigens presented on the surface of target cells by MHC class I molecules. Once a cytotoxic T cell binds to a target cell displaying an antigen it recognizes, it initiates a process to induce apoptosis, or programmed cell death, in the target cell. This is typically achieved through the release of cytotoxic molecules like perforin and granzymes. Perforin forms pores in the target cell membrane, allowing granzymes to enter and trigger the apoptotic cascade.

Cytotoxic T cells are generated in the thymus from precursor T cells. Their development involves a complex maturation process that includes positive and negative selection to ensure they can recognize foreign antigens effectively and do not attack the body's own healthy cells. Upon activation, usually by helper T cells and antigen-presenting cells, cytotoxic T cells proliferate and differentiate into effector cells that patrol the body, seeking out and destroying compromised cells. This ability to directly kill abnormal cells is vital for maintaining tissue homeostasis and preventing the spread of infection and disease.