Lesson 2 of 5 / Immune recognition and memory
Coordinate an adaptive response
How does a T cell recognise infection inside another cell?
In this lesson: Explain antigen-presenting cells, helper T cells and cytotoxic T cells.
About 7 min
The key ideaAntigen 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 appropriate antigen presented by infected cells and can trigger their death, reducing sites 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.
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.
Is this true? "T cells are the plasma cells that release antibodies."
Plasma cells are differentiated B cells; T cells have coordinating or cell-killing roles.