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Immunology · Immune cells

CD8⁺ cytotoxic T cell !

  • Cell

Cytotoxic T cells recognize viral or tumor peptides on MHC class I and kill the cell that shows them.

Updated 2026-11-01

What it is

CD8⁺ T cells recognize peptides from inside a cell presented on MHC class I, which almost every nucleated cell expresses. When a cell shows a foreign peptide, such as from a virus or a mutated tumor protein, the activated CD8 cell binds it and triggers its death by apoptosis. They are central to clearing viruses, controlling tumors and rejecting transplanted organs.

Key facts

  • Origin: lymphoid progenitor in the marrow; selected in the thymus on MHC class I.
  • Memory CD8 cells persist for years and respond faster on re-exposure.
  • Viruses and tumors can lower MHC class I to escape them, which is when NK cells take over.

Step by step

  1. A virus makes proteins inside an infected cell.
  2. The proteasome cuts them into peptides, which TAP carries into the endoplasmic reticulum.
  3. Peptides load onto MHC class I and travel to the cell surface.
  4. An activated CD8 T cell with a matching receptor binds the complex.
  5. It releases perforin and granzymes (or uses Fas ligand) and the infected cell dies by apoptosis.

Markers

  • CD3
  • CD8
  • TCR αβ
  • Perforin and granzymes
  • PD-1 when exhausted

Clinical notes

  • Atypical lymphocytes in infectious mononucleosis are mostly activated CD8 T cells responding to EBV-infected B cells.
  • Persistent infections and tumors exhaust CD8 cells (PD-1, CTLA-4); checkpoint inhibitors release this brake and can cause autoimmune side effects.
  • HLA class I (for example HLA-B27) presents peptides to CD8 cells and is linked to some diseases.
  • Low CD8 function contributes to severe viral infections in combined immunodeficiencies.

Test yourself

Think of the answer, then tap the question to see it.

Which MHC class presents antigen to CD8 T cells, and where do those peptides come from?

MHC class I, presenting peptides made inside the cell, such as viral or tumor proteins.

What are two ways a CD8 T cell kills its target?

Perforin and granzymes, and Fas ligand binding Fas on the target cell; both induce apoptosis.

What are the atypical lymphocytes of infectious mononucleosis?

Mostly activated CD8 T cells responding to Epstein-Barr virus-infected B cells.

Why do NK cells complement CD8 T cells?

When a virus or tumor lowers MHC class I to escape CD8 cells, NK cells kill the cells that lack MHC class I.

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← All of immunology

Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given.

Sources: 21.3 The Adaptive Immune Response: T lymphocytes and Their Functional Types (OpenStax, Rice University (Anatomy and Physiology 2e), 2022); Natural Killer Cells (British Society for Immunology, 2024); 18.4 Leukocytes and Platelets (OpenStax, Rice University (Anatomy and Physiology 2e), 2022).

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