Explain how antigen presentation by macrophages leads to the activation of T lymphocytes.

IB DP Biology Standard Level (2023 syllabus) — C3.2 Defence against disease · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Macrophages engulf pathogens by phagocytosis and digest them.
Fragments of the pathogen’s antigens are displayed on the macrophage surface as MHC‑II molecules (antigen presentation).
T lymphocytes with receptors complementary to the presented antigen bind to the MHC‑II/peptide complex.
This receptor–ligand interaction activates the T cell, causing clonal expansion and differentiation into helper or cytotoxic T cells.

Examiner tips

Use the phrase ‘antigen presentation’ and specify MHC‑II on macrophages. Show the sequence: phagocytosis → digestion → peptide–MHC display → T‑cell receptor binding → activation.
Mention clonal expansion and differentiation to highlight downstream effects.

Common mistakes

Confusing MHC‑I for macrophage presentation; MHC‑II is used by professional APCs. Omitting the step that the T cell receptor must recognise the peptide–MHC complex. Failing to mention clonal expansion or the two T‑cell lineages.

Mark scheme (4 marks)

Macrophages engulf pathogens by phagocytosis and digest them.
Fragments of the pathogen's antigens are displayed on the surface of the macrophage (antigen presentation).
T lymphocytes with complementary receptors bind to the presented antigen.
This binding activates the T lymphocyte, leading to clonal expansion and differentiation into helper T cells or cytotoxic T cells.

Key terms in this question

antigen presentation · macrophage · T lymphocyte · antigen