Phagocytosis

Where it happens

Phagocytosis happens wherever a phagocyte, a type of white blood cell, meets a pathogen in the blood or body tissues. It is part of the body's non-specific defence, so it acts against any pathogen in the same way, without needing to recognise it first.

The main phagocytes are neutrophils, which circulate in the blood and are the first to arrive at a wound or infected tissue, and macrophages, which settle in the tissues, lymph nodes, spleen, liver and lungs. Phagocytes squeeze out of capillaries through gaps between the endothelial cells, a movement that is easier when an inflamed area has widened, leaky capillaries.

The pus that collects in an infected wound is largely dead phagocytes, dead bacteria and tissue fluid, which is direct evidence of phagocytosis at work.

Inputs and outputs

• Input: a pathogen or foreign particle, such as a bacterium, a dead cell or debris, in the blood or tissue.
• Input: chemical signals released by damaged tissue and by the pathogen, which attract phagocytes to the site.
• Input: energy from ATP, used to extend the membrane and move the vacuole.
• Input: digestive enzymes stored in lysosomes inside the phagocyte.
• Output: harmless digested fragments, some released from the cell and some displayed on its surface.
• Output: a lower number of pathogens at the site, which gives lymphocytes time to mount the specific response.

The steps

1. Damaged tissue and the pathogen release chemicals that attract phagocytes, which move towards the site of infection.
2. The phagocyte recognises the pathogen as foreign when its surface receptors bind to the pathogen's surface molecules.
3. The phagocyte's cell membrane flows outward, forming extensions that surround the pathogen on all sides.
4. The extensions meet and fuse, enclosing the pathogen in a membrane-bound vacuole inside the cytoplasm.
5. Lysosomes inside the phagocyte move to the vacuole and fuse with it, releasing digestive enzymes into the vacuole.
6. The enzymes break down the pathogen into small, harmless molecules.
7. Useful products are absorbed into the cytoplasm and the remaining debris is expelled from the cell; a macrophage may also display fragments of the pathogen on its surface to alert lymphocytes.

Why it matters and how it is controlled

Phagocytosis provides a fast, non-specific line of defence that can act against many different pathogens as soon as they enter the body, without waiting for a specific response to develop. It works alongside barriers such as the skin and the specific immune response carried out by lymphocytes.

Phagocytosis is switched on by infection rather than running all the time. Chemicals from damaged cells and from the pathogen draw phagocytes to the site, and inflammation raises blood flow so more phagocytes arrive, which is why an infected area becomes red, warm and swollen.

The number of white blood cells in the blood also rises during infection, and a raised white cell count is one sign a doctor uses to detect infection. Phagocytosis also forms the bridge to the specific response: after digesting a pathogen, a macrophage can present pieces of its antigens to lymphocytes, which then produce antibodies against that particular pathogen.

How it is examined

You may be asked to describe the stages of phagocytosis using a diagram, to explain why phagocytosis is described as non-specific, or to compare phagocytosis with the specific immune response involving lymphocytes and antibodies.

Sequence questions give four or five labelled diagrams of a phagocyte and a bacterium out of order and ask you to arrange them and describe what happens at each stage; the words engulf, vacuole, lysosome, enzyme and digest are the expected key terms. Comparison tables ask you to set phagocytosis against the antibody response under headings such as speed, specificity, cells involved and memory.

Application questions describe pus in a wound or a raised white blood cell count and ask you to explain the observation. Marks depend on naming the lysosome as the source of enzymes and on stating that the pathogen is enclosed in a vacuole before digestion.

Common misconceptions

Worked exam-style question

Question. A student cut a finger while gardening. Two days later the cut was red, swollen and contained yellowish pus, and a blood test showed a higher than usual number of white blood cells.

(a) Name the type of white blood cell mainly responsible for the defence in the wound and the process it carries out. (b) Describe how this cell destroys a bacterium that has entered the wound.

(c) Explain what the pus consists of. (d) State two ways in which this defence differs from the response involving antibodies.

Model answer. (a) A phagocyte, carrying out phagocytosis. (b) The phagocyte is attracted by chemicals released at the wound and moves to the bacterium.

It extends its cell membrane around the bacterium and engulfs it, enclosing it in a vacuole. Lysosomes fuse with the vacuole and release digestive enzymes, which break down the bacterium into harmless products; the debris is then expelled.

(c) Pus is a mixture of dead phagocytes, dead bacteria and tissue fluid that collects at the site of infection. (d) Phagocytosis is non-specific, acting on any pathogen in the same way, while the antibody response is specific to one antigen; phagocytosis is immediate, whereas the antibody response takes days to build; phagocytosis leaves no memory cells, while the antibody response does.

Source:SRC-DSKP-EN

Frequently asked questions