# T lymphocytes and cell-mediated immunity

> Biology · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9700-u11-t-lymphocytes-and-cell-mediated/

This sub-topic covers the different types of T lymphocytes, their roles in the specific immune response, and the step-by-step sequence of events in cell-mediated immunity, which targets infected, cancerous, or foreign body cells.

**Prerequisites:** [Antigens and antigen presentation](https://www.owlsprep.com/study/cie-9700-u11-antigens-antigen-presenting-cells/); [Non-specific immune responses](https://www.owlsprep.com/study/cie-9700-u11-non-specific-immune-response/)

## Learning objectives

- Distinguish between the different functional types of T lymphocytes
- Describe the process of antigen presentation and T cell activation
- Outline the sequence of events in cell-mediated immunity
- Explain the role of cell-mediated immunity in the specific immune response

## Types and Functions of T Lymphocytes

**T Lymphocyte (T Cell)** — A type of lymphocytic white blood cell that matures in the thymus gland, mediates cell-mediated immunity. Each T cell has a unique T cell receptor (TCR) that recognizes specific antigen-MHC complexes.

*Notation:* T cell

*Example:* A cytotoxic T cell specific for a viral antigen will only recognize cells infected with that same virus.

There are three main functionally distinct types of T cells involved in the specific immune response, each with a specialized role:

- **Helper T cells (CD4+ T cells)**: Secrete signaling molecules called cytokines that activate B cells, cytotoxic T cells and macrophages
- **Cytotoxic T cells (CD8+ T cells)**: Directly kill infected body cells, cancer cells and transplanted foreign cells
- **Memory T cells**: Long-lived cells that persist after infection, enabling a faster secondary response to re-infection

**Worked example:** A student claims all T cells kill infected cells. Evaluate this statement.

1. The statement is incorrect, as not all T cells have a cytotoxic function.
2. Correct the statement by outlining the role of each T cell type:
3. Helper T cells do not kill cells; they secrete cytokines that activate other immune cells like B cells and cytotoxic T cells.
4. Only cytotoxic T cells have the function of killing infected, cancerous or foreign body cells.
5. Memory T cells do not kill cells during a primary response; they persist to enable a rapid secondary response upon re-exposure.

## Antigen Presentation and T Cell Activation

Unlike B cells, T cells cannot recognize free, unprocessed antigens. For a T cell to become activated, antigen fragments must be processed and presented on the surface of an antigen-presenting cell (APC) such as a dendritic cell, macrophage, or infected body cell.

**MHC Proteins** — Major Histocompatibility Complex (MHC) proteins are cell surface molecules that bind antigen fragments and display them to T cells. MHC Class I is found on all nucleated cells, MHC Class II only on professional APCs.

> **MHC Class Memory Hook**
>
> MHC I = Cytotoxic (I Kill), MHC II = Helper (II Help)

**Worked example:** Explain why free flu viral antigens in blood cannot activate T cells.

1. Recall the core property of T cell antigen recognition:
2. T cell receptors only bind to antigen fragments bound to MHC proteins on the surface of another cell, not free antigens.
3. Free viral antigens in plasma are not processed or bound to MHC proteins, so they cannot be recognized by T cell receptors.
4. For activation, viral antigens must be phagocytosed by an APC, processed, and displayed on MHC proteins to activate T cells.

## Sequence of Events in Cell-Mediated Immunity

After activation, T cells undergo clonal selection and expansion to produce a population of cloned T cells specific for the activating antigen, leading to a complete cell-mediated response. The full sequence is as follows:

1. APCs engulf pathogens, process antigens, and display fragments on MHC proteins
2. A complementary naive helper T cell binds to the antigen-MHC II complex and becomes activated
3. Activated helper T cells undergo clonal expansion and secrete cytokines to activate cytotoxic T cells
4. Complementary cytotoxic T cells bind to antigen-MHC I on infected cells and are activated by helper T cell cytokines
5. Activated cytotoxic T cells form clones of active cytotoxic T cells and memory T cells
6. Active cytotoxic T cells release perforin and granzymes to induce apoptosis in target cells
7. Memory T cells persist in lymphoid tissue for long-term immunity

**Worked example:** Outline the cell-mediated response to a measles-infected body cell.

1. Viral proteins from replicating measles virus are processed into small antigen fragments inside the infected cell.
2. Antigen fragments are bound to MHC I proteins and displayed on the infected cell surface.
3. A complementary cytotoxic T cell with a matching T cell receptor binds to the antigen-MHC I complex.
4. Activation is completed when cytokines from activated helper T cells stimulate the cytotoxic T cell.
5. The activated cytotoxic T cell divides by mitosis to produce a clone of active cytotoxic T cells and memory T cells.
6. Active cytotoxic T cells release perforin to form pores in the infected cell membrane, and granzymes trigger apoptosis, killing the cell before new viruses are released.
7. Memory T cells remain in circulation to respond rapidly if re-infection with measles occurs.

## Common pitfalls

- **Wrong:** Claiming T cells recognize free antigens the same way B cells do
  - Why it fails: Confusion between T cell and B cell antigen recognition mechanisms
  - Correct: State that T cells only recognize processed antigens presented on MHC proteins of other cells
- **Wrong:** Stating cytotoxic T cells engulf and digest infected whole cells
  - Why it fails: Confusion between phagocyte function and cytotoxic T cell function
  - Correct: State that cytotoxic T cells trigger apoptosis in target cells via cytotoxic chemicals, they do not phagocytose whole cells
- **Wrong:** Mixing up the roles of helper and cytotoxic T cells in answers
  - Why it fails: Common memory error between the two main T cell types
  - Correct: Recall helper T cells activate other immune cells via cytokines; only cytotoxic T cells kill abnormal/infected cells
- **Wrong:** Claiming cell-mediated immunity targets free pathogens in blood
  - Why it fails: Confusion between cell-mediated and humoral immunity functions
  - Correct: State that cell-mediated immunity targets infected/abnormal body cells; humoral immunity targets free extracellular pathogens

## Cheatsheet

| T Cell Type | Core Function | Key Marker | Recognizes |
| --- | --- | --- | --- |
| Helper T cell | Secrete cytokines to activate other immune cells | CD4+ | Antigen on MHC II |
| Cytotoxic T cell | Kill infected, cancerous, foreign target cells | CD8+ | Antigen on MHC I |
| Memory T cell | Enable rapid secondary immune response | CD4+ or CD8+ | Specific antigen-MHC complex |

## What's next

Understanding T lymphocytes and cell-mediated immunity is foundational for all further learning about immunity and immune system function. This branch of specific immunity works alongside humoral immunity (mediated by B cells) to provide complete protection against intracellular pathogens like viruses, intracellular bacteria, and parasites. Cell-mediated immunity is also responsible for cancer immune surveillance and the rejection of transplanted organs, two topics that are commonly tested alongside core T cell biology in CIE A-Level exams. Next, you can explore immunological memory, autoimmune disorders, or compare cell-mediated immunity to the humoral response to solidify your understanding of the full specific immune system.

- [Immune Memory and Vaccination](https://www.owlsprep.com/study/cie-9700-u11-immune-memory-and-vaccination/)
- [Monoclonal Antibodies](https://www.owlsprep.com/study/cie-9700-u11-monoclonal-antibodies/)

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