(e) (i) the structure and mode of action of phagocytes

The idea

Phagocytes are white blood cells that engulf and digest pathogens by phagocytosis. Neutrophils have a lobed nucleus and granular cytoplasm; macrophages are larger, with a rounded or kidney-shaped nucleus, and act as antigen-presenting cells. In phagocytosis the phagocyte is attracted to chemicals from the pathogen, binds it, and the cell-surface membrane invaginates to enclose the microorganism in a vesicle called a phagosome. Lysosomes fuse with the phagosome and release hydrolytic enzymes (lysozymes) that digest the pathogen. Soluble digestion products are absorbed into the cytoplasm. In macrophages, antigens from the digested pathogen are displayed on the cell-surface membrane, helping to activate other immune responses. MCQ items test recognition of these structural features and the correct ordered sequence of phagocytosis, so learn the stages and the organelle roles precisely.

What earns the marks

• Phagocytes are white blood cells of the immune system that carry out phagocytosis.
• Neutrophils have a lobed (multi-lobed) nucleus and granular cytoplasm; macrophages are larger with a rounded or kidney-shaped nucleus.
• The phagocyte is attracted to the pathogen by chemicals it releases and binds to it.
• The cell-surface membrane invaginates (engulfs) the pathogen to form a phagosome vesicle.
• Lysosomes fuse with the phagosome and release hydrolytic enzymes (lysozymes) that digest the pathogen.
• Digested soluble products are absorbed into the phagocyte's cytoplasm.
• Macrophages act as antigen-presenting cells, displaying pathogen antigens on their cell-surface membrane.

Common mistakes and how to put them right

• Thinking the pathogen is digested by enzymes in the cytoplasm before being enclosed.

  Put it right: Correct that engulfment into a phagosome happens first, then lysosomes fuse and release enzymes.

• Confusing phagocytes with lymphocytes.

  Put it right: Correct that phagocytes engulf pathogens non-specifically, whereas lymphocytes such as B and T cells give specific responses.

• Believing lysosomes are the same as the phagosome.

  Put it right: Correct that the phagosome is the vesicle containing the pathogen, and lysosomes are separate organelles that fuse with it.

• Assuming all phagocytes present antigens.

  Put it right: Correct that macrophages are the main antigen-presenting phagocytes, while neutrophils mainly destroy pathogens.

Exam tips

• Match each structure to its function: lobed nucleus and granules for neutrophils, antigen presentation for macrophages.
• Eliminate options that place digestion before engulfment or that name the wrong organelle.
• Check whether an option describes specific (lymphocyte) or non-specific (phagocyte) immunity before selecting.