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2.5.3Cell Structure

Phagocytosis — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 32 of 47 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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2.5.3 Phagocytosis — AQA A Level Biology 7402 notes: Describe the stages of phagocytosis and explain how phagocytes destroy pathogens and present antigens to start the specific immune response.Phagocytosis is the engulfing and digestion of pathogens by cells called phagocytes.It …

  • neutrophils
  • macrophages
  • Chemotaxis
  • Recognition and attachment
  • Engulfment
  • Fusion with lysosomes

Section 1 of 9What phagocytes are

Phagocytosis is the engulfing and digestion of pathogens by cells called phagocytes.

It is a non-specific defence because phagocytes will engulf many types of pathogen.
Neutrophils
The main phagocytes are neutrophils, which are short-lived and are the first to arrive at the site of infection, and macrophages, which are larger, longer-lived cells formed from monocytes.
Both are white blood cells made in the bone marrow.
Neutrophils have a multi-lobed nucleus and many lysosomes in the cytoplasm; macrophages have a larger, rounded or bean-shaped nucleus.

Phagocytes are found in blood and tissue fluid, and in the lymph nodes, spleen and liver, where macrophages remove pathogens and dead cells.

Section 2 of 9The stages of phagocytosis

1Chemotaxis
Chemotaxis chemicals released by pathogens, or by damaged cells, attract the phagocyte, which moves towards them. Neutrophils leave the capillaries by squeezing between the cells of the capillary wall.
2Recognition and attachment
Recognition and attachment receptors on the phagocyte's surface bind to antigens on the pathogen. In many cases the pathogen has first been coated with antibodies (opsonisation), which makes it more easily recognised.
3Engulfment
Engulfment the cell-surface membrane extends around the pathogen, enclosing it in a vesicle called a phagosome.
4Fusion with lysosomes
Fusion with lysosomes lysosomes, which contain hydrolytic enzymes, fuse with the phagosome to form a phagolysosome.
5Digestion
Digestion lysozymes and other hydrolases break down the pathogen. Useful products are absorbed into the cytoplasm and the waste is egested.
6Antigen presentation
Antigen presentation in macrophages, antigens from the pathogen are placed on the cell-surface membrane, turning the macrophage into an antigen-presenting cell.

Section 3 of 9Linking to the specific response

A macrophage that presents the antigen is the link between non-specific and specific immunity.

T helper cell
A T helper cell with a receptor complementary to the antigen binds to the antigen-presenting cell.
This stimulates the T helper cell to divide by mitosis and to release cytokines, which activate B cells and cytotoxic T cells.
In this way, phagocytosis not only removes the pathogen but also starts the adaptive response which will give long-term protection.
FeatureNeutrophilMacrophage
LifespanShort (hours to days)Long (months)
ArrivalFirst on siteLater
Presents antigensGenerally notYes
NucleusLobedLarge, rounded

Section 4 of 9Inflammation and pus

At the site of infection, damaged tissue and mast cells release chemicals such as histamine, which increase blood flow and make capillaries more permeable. Fluid and neutrophils enter the tissue, leading to swelling, redness and heat. Dead neutrophils, damaged tissue and pathogens form pus. This is part of the non-specific response and helps phagocytes reach the pathogen.

Some bacteria avoid phagocytosis by having a capsule, or by releasing toxins that kill phagocytes, or by living inside phagocytes. This is why antibodies, which bind to pathogens and promote phagocytosis, are useful.

Section 5 of 9Exam links

Use the correct terminologyengulf, phagosome, lysosome, hydrolytic enzymes, antigen-presenting cell.

Compare with other uses of the lysosome (autolysis, digestion of worn organelles).
Endocytosis
Remember that phagocytosis is a form of endocytosis and needs ATP.
In longer questions about the immune response, begin with phagocytosis and then carry on to T cells and B cells, using the antigen presentation as the transition.

Section 6 of 9Phagocytosis in context: defences before it

Phagocytes are only one part of the non-specific response.

1
The skin forms a physical barrier, and sebum and sweat make it slightly acidic.
2
Mucus in the airways traps microorganisms, and cilia sweep it upwards to be swallowed, where stomach acid (pH 1-2) kills most microbes.
3
Lysozyme in tears, saliva and mucus digests bacterial cell walls.
4
Normal gut flora competes with pathogens.
5
If a pathogen passes these barriers, phagocytes are the next line of defence.
6
Remember that phagocytosis is also used to clear up dead and damaged cells; macrophages in the spleen remove old red blood cells.
7
It is therefore not only an immune process, but is also part of tissue maintenance.
8
In practical terms a question may give a micrograph of a neutrophil, and ask you to identify it by its multi-lobed nucleus, or ask you to calculate the actual size of a bacterium that was engulfed using magnification, so revise how to use the formula size = image size / magnification.

Section 7 of 9Exam-style questions

  1. Describe how a phagocyte destroys a bacterium. [5]5 marks

  2. Explain how a macrophage helps to start the specific immune response. [3]3 marks

  3. A patient has a low neutrophil count. Suggest why the patient is more likely to suffer bacterial infections. [2]2 marks

Section 8 of 9Key terms

TermMeaning
PhagocytosisEngulfing and digestion of pathogens or debris by a cell
NeutrophilShort-lived phagocyte that arrives first at infection sites
MacrophageLarge, long-lived phagocyte that also presents antigens
PhagosomeVesicle containing the engulfed pathogen
LysosomeOrganelle containing hydrolytic enzymes
OpsonisationCoating of a pathogen with antibodies to help phagocytosis
Antigen-presenting cellCell displaying foreign antigen on its surface for T cells

Section 9 of 9Exam technique

1
Saying lysosomes engulf the pathogen - the cell membrane engulfs it; lysosomes release enzymes into the phagosome.
2
Confusing phagocytosis with antibody production - phagocytes do not make antibodies.
3
Stating that phagocytosis is specific - it is non-specific.
4
Forgetting that antigen presentation by the macrophage is what links to T cells.
5
Writing 'the phagocyte eats the bacteria' without describing the vesicle and enzymes.

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