T lymphocytes and the cellular responseAQA A-Level Biology: Revision notes
Section 1
The cellular response: the big picture
The cellular response is the part of the immune response that involves T lymphocytes (T cells). T cells mature in the thymus and each has a receptor on its surface with a specific shape. The response deals especially with cells infected with viruses, abnormal body cells and foreign cells, which antibodies cannot reach.
The main cells in this subtopic are antigen-presenting cells, helper T cells (TH cells) and cytotoxic T cells (TC cells). You do not need to know about other types of T cell.
The cellular response targets cells that carry antigens on their surface, such as infected cells, because pathogens inside cells are protected from antibodies.
Section 2
Antigen-presenting cells
A pathogen's antigens must be shown to T cells before they respond. An antigen-presenting cell displays the antigen on its surface membrane.
- A phagocyte (such as a macrophage) engulfs and digests a pathogen, then presents fragments of its antigens on its surface.
- Infected body cells also display viral antigens on their surface, as do cancer cells.
This lets the T cell with a complementary receptor find its target.
Macrophages do not make antibodies, and they do not kill T cells. They engulf the pathogen and present its antigens.
Section 3
Helper T cells
Each helper T cell (TH cell) has a receptor of specific shape. Only a TH cell with a receptor complementary to the presented antigen binds to the antigen-presenting cell and is activated.
The activated TH cell divides by mitosis to form a clone of identical TH cells (clonal expansion). The clone releases cytokines, chemical signals that stimulate:
- cytotoxic T cells to divide and kill infected cells
- B cells to divide and secrete antibodies
- phagocytes to carry out more phagocytosis
Helper T cells do not destroy pathogens themselves. Their role is to coordinate and stimulate other cells.
Section 4
Cytotoxic T cells
Cytotoxic T cells (TC cells) kill infected, abnormal or foreign cells.
- A TC cell with a receptor complementary to the antigen displayed on the infected cell binds to it.
- Stimulated by cytokines from TH cells, it divides to form a clone of TC cells.
- TC cells release perforin, which makes holes in the target cell's membrane, so the cell dies.
Killing the infected cell stops it releasing new virus particles.
Cytotoxic T cells kill the infected host cell, not the free virus particles in the blood.
Section 5
Putting it together
Sequence for the cellular response:
- Phagocyte engulfs the pathogen and presents its antigens.
- A TH cell with a complementary receptor binds and is activated.
- The TH cell divides by mitosis to form a clone and releases cytokines.
- Cytokines stimulate TC cells, B cells and phagocytes.
- TC cells bind to infected cells and kill them with perforin.
The response is specific, because only T cells with complementary receptors are activated.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on T lymphocytes and the cellular response
- A researcher infected mice with a virus and later examined their immune cells. Macrophages that had engulfed viral particles were found to have fragments of viral protein on their surface. Only a small proportion of the helper T cells in each mouse became active and divided.Explain why only certain helper T cells are activated in response to the viral antigen.2 marks
- Mice were genetically engineered so that they could not produce helper T cells (TH cells). They were infected with a virus, alongside normal mice. Compared with the normal mice, the engineered mice produced few cytotoxic T cells and few antibodies, and the concentration of virus in their tissues stayed high.Explain why the concentration of virus stayed high in the engineered mice.2 marks
- A virus infects liver cells. A scientist mixed cytotoxic T cells from an infected mouse with two cultures: liver cells infected with the virus, and uninfected liver cells. Only the infected liver cells were killed.Explain why the cytotoxic T cells killed only the infected liver cells.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).