T lymphocytes and cell-mediated immunity
Biology· 2022-2024 Syllabus, 11.4(d)· 18 min read
1. Types and Functions of T Lymphocytes★★☆☆☆⏱ 5 min
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.
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
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.
2. Antigen Presentation and T Cell Activation★★★☆☆⏱ 6 min
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.
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.
3. Sequence of Events in Cell-Mediated Immunity★★★☆☆⏱ 7 min
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:
APCs engulf pathogens, process antigens, and display fragments on MHC proteins
A complementary naive helper T cell binds to the antigen-MHC II complex and becomes activated
Activated helper T cells undergo clonal expansion and secrete cytokines to activate cytotoxic T cells
Complementary cytotoxic T cells bind to antigen-MHC I on infected cells and are activated by helper T cell cytokines
Activated cytotoxic T cells form clones of active cytotoxic T cells and memory T cells
Active cytotoxic T cells release perforin and granzymes to induce apoptosis in target cells
Memory T cells persist in lymphoid tissue for long-term immunity
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.
4. Common Pitfalls
Wrong move:
Claiming T cells recognize free antigens the same way B cells do
Why:
Confusion between T cell and B cell antigen recognition mechanisms
Correct move:
State that T cells only recognize processed antigens presented on MHC proteins of other cells
Wrong move:
Stating cytotoxic T cells engulf and digest infected whole cells
Why:
Confusion between phagocyte function and cytotoxic T cell function
Correct move:
State that cytotoxic T cells trigger apoptosis in target cells via cytotoxic chemicals, they do not phagocytose whole cells
Wrong move:
Mixing up the roles of helper and cytotoxic T cells in answers
Why:
Common memory error between the two main T cell types
Correct move:
Recall helper T cells activate other immune cells via cytokines; only cytotoxic T cells kill abnormal/infected cells
Wrong move:
Claiming cell-mediated immunity targets free pathogens in blood
Why:
Confusion between cell-mediated and humoral immunity functions
Correct move:
State that cell-mediated immunity targets infected/abnormal body cells; humoral immunity targets free extracellular pathogens
5. Quick Reference 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 |
6. Frequently Asked
What is the difference between cell-mediated and humoral immunity?
Cell-mediated immunity targets infected/abnormal body cells via T cells, while humoral immunity targets free extracellular pathogens via antibodies produced by B cells.
Can T cells recognize free antigens?
No, T cells can only recognize processed antigen fragments presented on MHC proteins on the surface of other cells, unlike B cells which can bind free native antigens directly.
Going deeper
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.