International A Level Biology / Immunity

Phagocytosis revision notes

Edexcel International A Level Biology (YBI11), chapter 18: Immunity, specification point 6.8. Free lesson.

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Phagocytosis is how neutrophils and macrophages engulf and digest pathogens. Macrophages also present pathogen antigens to T helper cells, helping to start the specific immune response.

How neutrophils and macrophages differ

The two main phagocytes are neutrophils and macrophages. A neutrophil has a lobed nucleus, with several parts joined by thin strands. A macrophage has one large, rounded or bean-shaped nucleus. Macrophages develop from monocytes, which leave the blood and settle in tissues. Neutrophils are smaller, are the commonest white blood cells, and are mostly in the blood. They are usually the first to reach an infection. Macrophages are much larger and are mostly found in tissues. Neutrophils live for a few days and die after engulfing pathogens. Dead neutrophils and bacteria can build up as…

How phagocytes recognise and engulf a bacterium

Chemicals from pathogens and damaged cells draw phagocytes towards an infection. Receptors on the phagocyte then bind to foreign molecules on a bacterium. The cell membrane flows around it and encloses it by endocytosis. The enclosed bacterium sits inside a vesicle in the cytoplasm called a phagosome. A lysosome is a small sac containing hydrolytic enzymes. Its membrane keeps the enzymes separate from the rest of the cell.

How lysosomes digest the pathogen

Lysosomes move towards the phagosome and fuse with it. Their hydrolytic enzymes enter the phagosome and break down the bacterium. The enzymes stay enclosed by membranes, which helps prevent them from digesting the phagocyte itself. Soluble products of digestion pass into the cytoplasm. Any material left over leaves the cell by exocytosis.

How macrophages present antigens

After digestion, a macrophage keeps some of the pathogen's antigens, often pieces of its proteins. MHC proteins hold these antigens on the macrophage's surface. This makes the macrophage an antigen-presenting cell (APC). The macrophage presents the antigen to a T helper cell. A T helper cell binds when its receptor is complementary to the antigen. This starts the specific immune response. Before antigen presentation, the response is non-specific; afterwards, it is aimed at that antigen.

Which cells engulf pathogens?

Neutrophils and macrophages engulf pathogens by phagocytosis. T killer cells are not phagocytes: they bind to infected body cells and destroy them. Keep the roles distinct: phagocytes engulf pathogens, while T killer cells destroy infected body cells.