What is an antigen-presenting cell?

A cell that displays foreign antigens on its surface so that T cells can recognise them.

Give two examples of cells that present antigens.

A macrophage (phagocyte) that has engulfed a pathogen; a virus-infected body cell.

What determines which helper T cell is activated?

Only the TH cell with a receptor complementary to the presented antigen binds and is activated.

What does an activated helper T cell do first?

It divides by mitosis to form a clone of identical TH cells.

Chemical signals released by TH cells that stimulate cytotoxic T cells, B cells and phagocytes.

Name three cell types stimulated by helper T cells.

Cytotoxic T cells, B cells and phagocytes.

What is the role of cytotoxic T cells?

To kill infected, abnormal and foreign cells.

How do cytotoxic T cells kill target cells?

They release perforin, which makes holes in the target cell's membrane.

Why can antibodies not remove a virus inside a cell?

Antibodies cannot cross the cell surface membrane.

Why is the cellular response specific?

Only T cells with receptors complementary to the antigen are activated.

Why do helper T cells not kill pathogens themselves?

Their role is to release cytokines that stimulate the cells that do the killing.

T lymphocytes and the cellular responseAQA A-Level Biology: Flashcards

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What is the cellular response?

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What is the cellular response?: The immune response involving T lymphocytes.
What is an antigen-presenting cell?: A cell that displays foreign antigens on its surface so that T cells can recognise them.
Give two examples of cells that present antigens.: A macrophage (phagocyte) that has engulfed a pathogen; a virus-infected body cell.
What determines which helper T cell is activated?: Only the TH cell with a receptor complementary to the presented antigen binds and is activated.
What does an activated helper T cell do first?: It divides by mitosis to form a clone of identical TH cells.
What are cytokines?: Chemical signals released by TH cells that stimulate cytotoxic T cells, B cells and phagocytes.
Name three cell types stimulated by helper T cells.: Cytotoxic T cells, B cells and phagocytes.
What is the role of cytotoxic T cells?: To kill infected, abnormal and foreign cells.
How do cytotoxic T cells kill target cells?: They release perforin, which makes holes in the target cell's membrane.
Why can antibodies not remove a virus inside a cell?: Antibodies cannot cross the cell surface membrane.
Why is the cellular response specific?: Only T cells with receptors complementary to the antigen are activated.
Why do helper T cells not kill pathogens themselves?: Their role is to release cytokines that stimulate the cells that do the killing.

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

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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).