Lesson 2 of 5 / Immune recognition and memory
Coordinate an adaptive response
How does a T cell recognise infection inside another cell?
I can explain antigen-presenting cells, helper T cells and cytotoxic T cells.
I can explain B, T, antigen-presenting and memory cells in primary/secondary responses.Syllabus
Syllabus 9477, Extension A(b). Explain B, T, antigen-presenting and memory cells in primary/secondary responses.
About 7 min
Make a guess. You are not marked.
A virus is hiding inside a body cell. Can a T cell detect it there?
Show the answer
It can, from viral peptides shown on MHC.
Infected cells display fragments of viral proteins on MHC molecules. T-cell receptors recognise the peptide-MHC complex.
The key idea
Antigen presentation links cellular evidence of infection to specific T-cell activation and coordinated responses.
From recognition to response
What exactly does the T-cell receptor recognise?
A professional antigen-presenting cell takes up material and processes protein into peptides. This model shows material from outside the cell entering an internal compartment.
Brown triangle = presented peptide; teal = MHC; purple fork = T-cell receptor. Helper recognition typically involves MHC II and cytotoxic recognition MHC I; the simplified shapes show the principle rather than molecular structures. B-cell receptors can instead bind intact antigen directly.
Explanation
Antigen-presenting cells such as dendritic cells take up material, process proteins and display peptide fragments with cell-surface major histocompatibility complex molecules. This makes information about a pathogen available to T cells in an appropriate activation context.
A helper T cell recognises a suitable presented peptide through its receptor and, with required additional signals, becomes activated and proliferates. Its signals help coordinate other cells, including B cells and cytotoxic T cells. A helper T cell does not recognise every free antigen simply because it is nearby.
Cytotoxic T cells recognise antigen presented on the surface of infected cells. They then trigger apoptosis of the infected cell. Apoptosis is programmed cell death: the infected cell dismantles itself in a controlled way, which removes a site of viral production. This differs from antibody action on extracellular targets. Destroying an infected host cell can protect the organism even though it sacrifices that cell.
B cells can bind intact antigen through their receptors, internalise it and present fragments to helper T cells. Cooperation supports strong antibody responses and memory. The essential distinction is between recognising an antigen directly with a B-cell receptor and recognising a presented peptide with a T-cell receptor.
Name the two kinds of MHC molecule. Antigen-presenting cells (dendritic cells, macrophages and B cells) engulf antigen from outside the cell, break it into peptides and display them on MHC class II molecules. Helper T cells recognise these. Almost all nucleated cells display peptides made inside the cell, including viral proteins in an infected cell, on MHC class I molecules. Cytotoxic T cells recognise these.
A T-cell receptor recognises a specific peptide only when it is held in an MHC molecule, as a peptide-MHC complex; it does not bind free antigen. This is called MHC restriction. When a helper T cell's receptor binds its matching peptide-MHC class II complex, the helper T cell is activated, divides, and secretes cytokines. Cytokines are signalling proteins that stimulate the matching B cells to divide and become plasma cells and memory cells, and help activate cytotoxic T cells.
Step by step
- 1
Locate the antigen
Free extracellular material or a presented peptide?
- 2
Identify the responding cell
B-cell binding and T-cell recognition differ.
- 3
Name the output
Coordination, antibody production or infected-cell killing.
Worked example
Work through the evidence
Why can loss of helper T cells weaken both antibody and cell-mediated responses?
One way to explain it
Helper T-cell signals support activation and coordination of multiple lymphocyte pathways, so their loss can compromise B-cell responses and effective cytotoxic responses.
Why this answer works
- One coordinating cell type influences several branches.
- Helper T cells do not themselves secrete all antibodies.
Watch out for this
A student says: "T cells are the plasma cells that release antibodies." What is wrong with this?
Show the answer
Plasma cells are differentiated B cells; T cells have coordinating or cell-killing roles.
Check your understanding
Which is a role of a cytotoxic T cell?
Show the answer
Triggering apoptosis of an infected cell that presents the matching antigen
Apoptosis, programmed cell death, removes a cellular site of pathogen replication.
Quick recall
How does a T cell recognise infection inside another cell?