Explain how phagocytes destroy pathogens that have entered body tissues.

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)

Phagocytes are attracted to the site of infection by chemical signals such as cytokines and chemokines released by damaged or infected cells.
They engulf the pathogen by endocytosis, forming a phagosome.
The phagosome fuses with lysosomes to form a phagolysosome, where hydrolytic enzymes are released.
These enzymes digest the pathogen, destroying it; the breakdown products are either expelled or recycled by the phagocyte.

Examiner tips

Use the exact sequence: attraction → engulfment → fusion → digestion.
Mention the key terms: chemotaxis, phagosome, phagolysosome, hydrolytic enzymes.
Show the process in a clear, step‑by‑step order to earn all marks.

Common mistakes

Confusing phagocytosis with pinocytosis.
Omitting the role of lysosomes or hydrolytic enzymes.
Using vague phrases like ‘the cell kills the pathogen’ without detailing the mechanism.

Mark scheme (4 marks)

Phagocytes are attracted to the site of infection by chemical signals (chemotaxis/cytokines/chemokines) released by damaged or infected cells.
Phagocytes engulf pathogens by endocytosis, forming a phagosome (vacuole) around the pathogen.
Lysosomes fuse with the phagosome, releasing hydrolytic (digestive) enzymes into the phagolysosome.
The hydrolytic enzymes break down / digest the pathogen, destroying it; products may be expelled from the cell or used by the phagocyte.

Key terms in this question

phagocyte