Cytotoxic T cells (CTLs) are specialized white blood cells that form a component of the adaptive immune system. These cells function as “killer cells,” identifying and eliminating dangerous cells within the body. Their primary role involves recognizing and destroying cells compromised by internal changes or foreign invaders, ensuring only affected cells are removed while preserving healthy tissue.
How Cytotoxic T Cells Recognize Their Targets
Cytotoxic T cells identify their target cells through a sophisticated molecular process involving Major Histocompatibility Complex (MHC) Class I molecules. Nearly all nucleated cells continuously present small fragments of their internal proteins on their surface using these MHC Class I molecules. This display allows the immune system to monitor the cell’s internal state.
Cytotoxic T cells possess unique T-cell receptors (TCRs) on their surface, designed to inspect these presented protein fragments. If the TCR recognizes a protein fragment as abnormal or foreign, it signals that the cell is compromised. This mechanism allows CTLs to distinguish between healthy cells, which present normal “self” proteins, and cells with undesirable alterations or infections, presenting “altered-self” or foreign fragments.
Target: Cells Infected by Viruses
Cells infected by viruses represent a primary target for cytotoxic T cells. When a virus invades a host cell, it hijacks the cell’s machinery to produce its own viral proteins. Some of these newly synthesized viral proteins are degraded into smaller pieces within the cell. These viral fragments are then loaded onto MHC Class I molecules and transported to the cell surface.
The presence of these viral protein fragments on the cell’s surface signals infection. Cytotoxic T cells, patrolling the body, recognize these foreign viral fragments via their T-cell receptors. Upon recognition, the CTL eliminates the infected cell, preventing further viral replication and spread. This defense mechanism helps control infections caused by viruses like influenza, HIV, and Epstein-Barr virus.
Target: Cancerous Cells
Cancerous cells are another target for cytotoxic T cells, playing a role in the body’s defense against tumor development. As cells become cancerous, they often undergo genetic mutations, leading to the production of abnormal or altered proteins. These unique proteins, sometimes called tumor antigens or neoantigens, are not found in healthy cells.
Similar to viral fragments, these abnormal proteins within cancerous cells are processed and presented on the cell surface by MHC Class I molecules. Cytotoxic T cells recognize these “altered-self” proteins as indicators of cellular malfunction. When a CTL encounters a cancerous cell presenting such antigens, it destroys the malignant cell. This ongoing process, known as immune surveillance, helps identify and eliminate nascent tumors.
The Mechanism of Target Elimination
Once a cytotoxic T cell recognizes and binds to a compromised target cell, it initiates programmed cell death, known as apoptosis. This controlled destruction prevents the release of cellular contents that could trigger inflammation or damage surrounding healthy tissues. The CTL achieves this by releasing specialized cytotoxic molecules directly at the target cell.
Two primary molecules involved are perforin and granzymes. Perforin molecules are released first and form pores in the target cell’s outer membrane. Granzymes, a type of enzyme, then enter the target cell through these pores. Once inside, granzymes activate a cascade of enzymes that dismantle the cell from within, leading to its demise. This targeted approach ensures efficient removal of dangerous cells with minimal collateral damage.