U01.02.013 Cytotoxic T cell

Cytotoxic T cells (also known as CD8+ T cells) play a crucial role in the immune system’s response to infected, neoplastic (tumor), and foreign donor cells. Their primary function is to eliminate these harmful cells through apoptosis, ensuring the integrity of healthy tissues.

Mechanism of Action

Cytotoxic T cells recognize and destroy infected or abnormal cells through a highly regulated process:

Target Identification:
- Cytotoxic T cells possess CD8 receptors that specifically bind to Major Histocompatibility Complex (MHC) Class I molecules.
- MHC I molecules are expressed on the surface of all nucleated cells, displaying peptide fragments of intracellular pathogens like viruses.
Induction of Apoptosis:
- Once bound to MHC I on a virus-infected or neoplastic cell, cytotoxic T cells release cytotoxic granules.
- These granules contain proteins such as perforin and granzyme B:
  - Perforin: Forms pores in the target cell membrane, facilitating the entry of granzyme B.
  - Granzyme B: Induces programmed cell death (apoptosis) by activating caspases inside the infected or abnormal cell.
Elimination of Cells:
- The combination of pore formation and enzymatic activity efficiently destroys infected, cancerous, or grafted cells without damaging surrounding healthy tissues.

Table: Cytotoxic Granules and Their Functions

Protein	Function
Perforin	Creates pores in the target cell membrane
Granzyme B	Triggers apoptosis by activating caspases

Table: Cytotoxic Granules and Their Functions

Points to Remember

Cytotoxic T cells primarily target cells infected with viruses, cancerous cells, and transplanted cells.
They recognize these cells through the CD8 receptor, which binds to MHC I molecules presenting foreign peptides.
Cytotoxic granules contain perforin and granzyme B, which work together to induce cell death.
The process is highly specific, minimizing collateral damage to healthy cells.

Bibliography Reference Links

Abbas AK, Lichtman AH, Pillai S. Cellular and Molecular Immunology. 9th ed. Elsevier; 2017.
Janeway CA, Travers P, Walport M, et al. Immunobiology: The Immune System in Health and Disease. 5th ed. Garland Science; 2001.
Murphy K, Weaver C. Janeway’s Immunobiology. 9th ed. Garland Science; 2016.

USMLE Step 1 Review Series

Curriculum

U01.02.013 Cytotoxic T cell

Mechanism of Action

Table: Cytotoxic Granules and Their Functions

Points to Remember

Bibliography Reference Links

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