Human defence systems: phagocytosis

The idea

Phagocytosis is the process by which a white blood cell engulfs and digests a pathogen. The white blood cell detects chemicals released by the pathogen and moves towards it. The cell membrane surrounds the pathogen, taking it into the cytoplasm inside a vacuole. Digestive enzymes inside the white blood cell break down the pathogen, destroying it. This is a non-specific defence because the same process can destroy many different types of pathogen. A concrete example is a phagocyte engulfing a bacterium at the site of an infected cut. Phagocytosis is one of the ways white blood cells help to defend against pathogens, alongside antibody production and antitoxin production.

What earns the marks

• Phagocytosis is carried out by white blood cells such as phagocytes.
• The white blood cell engulfs the pathogen, taking it into the cell inside a vacuole.
• Digestive enzymes break down and destroy the pathogen inside the white blood cell.
• Phagocytosis is non-specific, so it can destroy a range of different pathogens.
• Phagocytosis is one of several white blood cell defence mechanisms, alongside antibody and antitoxin production.

Common mistakes and how to put them right

• Confusing phagocytosis with antibody production.

  Put it right: Phagocytosis involves engulfing and digesting a pathogen, whereas antibody production involves antibodies binding to antigens.

• Stating that the pathogen is killed outside the white blood cell.

  Put it right: The pathogen is taken inside the white blood cell and digested there.

• Describing phagocytosis as specific to one pathogen.

  Put it right: It is a non-specific defence that can destroy many types of pathogen.

Exam tips

• Sequence the process clearly: detection, engulfing, vacuole formation, enzyme digestion.
• Use the term vacuole when describing where the pathogen is contained inside the white blood cell.
• Link phagocytosis to the wider role of white blood cells in defending against pathogens.