Cytotoxic T lymphocytes (CTLs) are a type of T cell that are crucial for the immune system's ability to recognize and kill infected or cancerous cells.
Understanding Cytotoxic T Lymphocytes (CTLs)
CTLs, also known as killer T cells or CD8+ T cells, are specialized immune cells that play a critical role in adaptive immunity. They are particularly important for eliminating intracellular pathogens, such as viruses, and for controlling tumor growth.
How CTLs Work: A Step-by-Step Process
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Antigen Presentation: CTLs recognize antigens presented on the surface of other cells via Major Histocompatibility Complex class I (MHC-I) molecules. These antigens are typically fragments of proteins found inside the cell, such as viral proteins in an infected cell or abnormal proteins in a cancer cell.
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Activation: When a CTL encounters a cell displaying a matching antigen presented by MHC-I, it becomes activated. This activation process also requires co-stimulatory signals from the antigen-presenting cell.
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Target Cell Recognition: Once activated, CTLs can roam the body, scanning cells for the specific antigen they were activated against.
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Cytotoxic Action: Upon recognizing a target cell displaying the antigen, the CTL binds to it and releases cytotoxic granules. These granules contain proteins like perforin and granzymes.
- Perforin: Forms pores in the target cell's membrane, allowing granzymes to enter.
- Granzymes: Serine proteases that enter the target cell and activate caspases, initiating apoptosis (programmed cell death).
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Target Cell Death: The target cell undergoes apoptosis, effectively eliminating the source of the antigen (e.g., the virus-infected cell or the cancer cell) without causing significant inflammation or damage to surrounding tissues.
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CTL Proliferation and Memory: After killing a target cell, the CTL can detach and move on to kill other infected cells. Furthermore, some activated CTLs differentiate into memory CTLs, which provide long-lasting immunity. These memory cells can rapidly respond to subsequent encounters with the same antigen, providing faster and more effective protection.
Role in Immunity
CTLs are essential for controlling:
- Viral infections: They eliminate virus-infected cells, preventing the virus from replicating and spreading.
- Intracellular bacterial infections: They can target cells infected with bacteria that live inside cells.
- Tumor immunity: They can recognize and kill cancer cells that express abnormal proteins.
- Graft rejection: They can contribute to the rejection of transplanted organs if they recognize foreign antigens on the donor cells.
CTLs in Immunotherapy
Because of their ability to selectively kill infected or cancerous cells, CTLs are a major focus of immunotherapy research. Strategies to enhance CTL activity, such as adoptive cell transfer (ACT) therapy, are being developed to treat various diseases. In ACT, a patient's own T cells are engineered to express a receptor that specifically targets cancer cells. These engineered cells are then expanded in vitro and infused back into the patient to eradicate the tumor.
