# 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

## Mark scheme (4 marks)

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

## Key terms

- [antigen presentation](https://www.gradenine.co.uk/glossary/antigen-presentation)
- [macrophage](https://www.gradenine.co.uk/glossary/macrophage)
- [T lymphocyte](https://www.gradenine.co.uk/glossary/t-lymphocyte)
- [antigen](https://www.gradenine.co.uk/glossary/antigen)

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-antigen-presentation-by-macrophages-364e035b) · Published by Druglandscape Ltd.