Trace phagocytosis by white blood cells
Describe how neutrophils and macrophages engulf and digest pathogens by phagocytosis.
Section 11.1 · The immune system
AS Level · Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; human subject-teacher review and supervised practical evidence remain separate.
Before you begin
- Pathogens carry antigens, and specific receptors on lymphocytes can recognise complementary antigen shapes.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which cell structures fuse with a phagosome to aid digestion?
What you will learn
- Describe pathogen recognition and engulfment by phagocytes
- Explain digestion within a phagosome after lysosome fusion
Build the explanation
Neutrophils and macrophages are phagocytes. They recognise and bind to a pathogen, then extend their cell surface around it and engulf it into a membrane-bound vesicle called a phagosome. Lysosomes fuse with the phagosome and release digestive enzymes; the pathogen is broken down. Macrophages can also process pathogen antigens and present them at the cell surface, helping connect innate phagocytosis to a specific immune response.
Apply it carefully
Keep the sequence ordered: recognition/binding → engulfment → phagosome → lysosome fusion → enzymatic digestion. Phagocytosis is not the same as antibody production; antibodies are made by plasma cells derived from activated B-lymphocytes. A diagram may simplify cell shape, so describe the process rather than claiming a phagocyte digests every pathogen in the same way.
Pause and explain
Which two white blood-cell types act as phagocytes in this syllabus?
Worked example
What happens after a pathogen is enclosed in a phagosome?
Show the worked solution
- A lysosome fuses with the phagosome.
- Digestive enzymes break down the pathogen.
Answer A lysosome fuses with the phagosome and releases enzymes that digest the pathogen.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe phagocytosis from pathogen recognition to digestion.
Give me a hint
Name the vesicle and the organelle that supplies enzymes.
Compare my reasoning
- A phagocyte binds and engulfs a pathogen into a phagosome.
- A lysosome fuses with the phagosome; enzymes digest the pathogen.
A neutrophil or macrophage recognises and binds to a pathogen, engulfs it into a phagosome, and lysosomes then fuse with that vesicle. Lysosomal enzymes digest the pathogen.
Look for these in your work
- Recognition and engulfment stated
- Phagosome named
- Lysosome and digestion linked
A student says neutrophils secrete antibodies around bacteria. Correct the cell and process.
Give me a hint
Distinguish phagocytes from antibody-secreting plasma cells.
Compare my reasoning
- Neutrophils engulf pathogens by phagocytosis.
- Plasma cells derived from B-lymphocytes secrete antibodies.
Neutrophils are phagocytes: they engulf bacteria and digest them after lysosome fusion. Antibodies are secreted by plasma cells formed from activated B-lymphocytes.
Look for these in your work
- Neutrophil role corrected
- Antibody-producing cell named
A micrograph shows a macrophage with a membrane-bound vesicle containing material from a pathogen. Explain the likely next digestive step.
Give me a hint
Follow what fuses with the vesicle.
Compare my reasoning
- The vesicle is a phagosome.
- A lysosome fuses and supplies digestive enzymes.
The vesicle is consistent with a phagosome. A lysosome can fuse with it and release enzymes that digest the engulfed pathogen. The image alone does not show the full biochemical sequence.
Look for these in your work
- Phagosome identified
- Lysosome step stated
- Image evidence calibrated
Common mistakes
- Confusing phagocytes with antibody-secreting plasma cells
- Omitting the phagosome or lysosome fusion
- Assuming phagocytosis is specific antibody recognition
What can you explain now?
Which cell structures fuse with a phagosome to aid digestion?
Compare with the explanation
Lysosomes
Lysosomes contain enzymes that digest the engulfed pathogen.
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Make it stick
What is the sequence from phagocyte binding to pathogen digestion?
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