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Immunity: Question 1

Syllabus 11.1

Multiple choice AS 1 mark

A neutrophil in an infected wound comes into contact with a bacterium and destroys it by phagocytosis.

Which statement correctly describes how the neutrophil destroys the bacterium?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall the stages of phagocytosis

Phagocytosis is the process by which certain white blood cells (phagocytes), such as neutrophils and macrophages, engulf and destroy pathogens. The process happens in a defined sequence:

  1. The phagocyte is attracted to the pathogen, often by chemicals released from the site of infection.
  2. The phagocyte engulfs the pathogen by endocytosis: its cell-surface membrane folds around the pathogen, enclosing it in a vesicle called a phagosome inside the cytoplasm.
  3. A lysosome (a vesicle containing hydrolytic/digestive enzymes) fuses with the phagosome to form a phagolysosome.
  4. The hydrolytic enzymes (including lysozymes) inside the phagolysosome digest and break down the pathogen.
  5. Any useful breakdown products may be absorbed, and any remaining debris is removed from the cell.

This means the destruction of the bacterium happens inside the phagocyte, only after it has been engulfed and enclosed, and only after a lysosome has fused with the phagosome to supply the digestive enzymes.

Step 2: Evaluate each option

  • A: correct, this matches the sequence above exactly: engulfment into a phagosome, fusion with a lysosome, then digestion by hydrolytic enzymes.
  • B: incorrect. Phagocytes do not typically digest bacteria by releasing enzymes into the surrounding tissue fluid; digestion happens inside the cell, after engulfment, not outside it.
  • C: incorrect. Producing antibodies is the role of B-lymphocytes (and the plasma cells they form), not of neutrophils, which act non-specifically by engulfment rather than by making antibodies.
  • D: incorrect, ribosomes synthesise proteins; they do not act directly on an engulfed bacterium. It is the hydrolytic enzymes released from a lysosome, after fusing with the phagosome, that digest the bacterium.

Final answer

  • The neutrophil destroys the bacterium by engulfing it into a phagosome, which then fuses with a lysosome so that hydrolytic enzymes can digest it, option A.