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6.2.3Topic 6 · Immunity, Infection and Forensics

Non-specific Immune Responses — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 9 parts Lesson 11 of 19 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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6.2.3 Non-specific Immune Responses — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe the non-specific responses to infection, including inflammation, phagocytosis, fever and interferon, and explain how they lead on to the specific immune response.If a pathogen gets past th…

  • Phagocytes
  • neutrophils
  • macrophages
  • phagolysosome
  • antigen-presenting cells
  • interferons

Part 1 of 7Non-specific defences

If a pathogen gets past the surface barriers, the body responds at once with general defences that act against any pathogen.

Non-specific
These are non-specific: they do not depend on recognising a particular antigen and they act in the same way each time.
They include inflammation, phagocytosis, fever and the production of antiviral proteins.
Specific
They also help to start the specific response by T and B lymphocytes.

Part 2 of 7Phagocytes and phagocytosis

Phagocytes are white blood cells that engulf and digest pathogens. The two main types are neutrophils (short-lived cells with a lobed nucleus, first to arrive at an infection) and macrophages (larger cells that develop from monocytes, live longer and are found in tissues). The stages of phagocytosis are as follows.

Chemicals released by pathogens or damaged cells attract phagocytes (chemotaxis).
Receptors on the phagocyte bind to the pathogen, often helped by opsonins, which are antibodies or other proteins that coat the pathogen.
Phagosome
The phagocyte surrounds the pathogen with its membrane and engulfs it, forming a phagosome vesicle.
Phagolysosome
Lysosomes fuse with the phagosome, forming a phagolysosome, and release lysozyme and other digestive enzymes that hydrolyse the pathogen.
Antigen-presenting cells
The products are absorbed, and the remains are released.
  • Macrophages display fragments of the pathogen as antigens on their surface and become antigen-presenting cells.

Part 3 of 7Inflammation

Damaged tissue and mast cells release histamine and other signalling molecules (cytokines).

1
Histamine makes the walls of capillaries more permeable and causes vasodilation.
2
More blood flows to the area, bringing redness and heat.
3
Plasma leaks into the tissue, causing swelling (oedema) and delivering more antibodies, complement proteins and white blood cells.
4
Phagocytes move out of the capillaries into the tissue.
5
Pus is the collection of dead phagocytes, pathogens and tissue fluid.
6
Inflammation concentrates the response where it is needed, though too much can cause damage, as in allergies or arthritis.
7
Antihistamines reduce the effects of histamine.

Part 4 of 7Fever and interferon

Cytokines from phagocytes act on the hypothalamus and raise the body temperature set point, causing a fever. A higher temperature reduces the growth rate of many pathogens and may speed up the activity of immune cells. Very high fever is dangerous because enzymes may denature.

Virus-infected cells release interferons, signalling proteins that bind to receptors on neighbouring cells and trigger them to make enzymes that block viral protein synthesis and degrade viral RNA. This limits the spread of the virus while the specific response develops.

Part 5 of 7Comparison with the specific response

Non-specificSpecific
Immediate (minutes to hours)Slower, days for the first response
Same against all pathogensTargets a particular antigen
No memoryMemory cells give a faster second response
Phagocytes, inflammation, fever, interferonT cells, B cells, antibodies

Antigen-presenting macrophages link the twothey show pathogen antigens to T helper cells, as explained in the following lessons.

Part 6 of 7Data and exam skills

You may be shown a graph of the numbers of neutrophils in the blood after an infection, or a table comparing temperature with pathogen growth.

Data and exam skills
Describe the pattern with numbers, for example the neutrophil count rose from 4 to 12 × 109 per dm3 within a few hours,
  • then fell as the infection cleared.
Explain the changes in terms of chemotaxis and release of cells from the bone marrow.
When asked about inflammation, make sure you state the sequence: damage, histamine release, vasodilation and increased permeability, more fluid and phagocytes at the tissue.

Remember also that the complement system, a group of plasma proteins, can be activated by pathogens or antibodies.

Data and exam skills
Complement proteins coat pathogens, attracting phagocytes, and may assemble into a membrane attack complex that makes holes in bacterial membranes.
Platelets and clotting seal wounds and so prevent entry of microorganisms.
Natural killer cells, a type of lymphocyte, destroy abnormal and virus-infected cells without needing a specific antigen,
  • which gives another non-specific defence.

Part 7 of 7Exam-style questions

  1. Describe how a phagocyte destroys a bacterium. [4]4 marks

  2. Explain how histamine causes the swelling seen at a site of infection. [3]3 marks

  3. Explain how interferon helps to protect cells from viral infection. [3]3 marks

Key terms

TermMeaning
PhagocytosisEngulfing and digesting of particles or cells by a cell.
NeutrophilA short-lived phagocytic white blood cell with a lobed nucleus.
MacrophageA large phagocyte that can present antigens.
HistamineA chemical released from damaged tissue that causes inflammation.
InterferonA protein released by virus-infected cells that protects neighbouring cells.
CytokineA signalling protein released by cells of the immune system.
OpsoninA molecule that coats a pathogen to make phagocytosis easier.
Antigen-presenting cellA cell that displays antigens on its surface to T cells.

Avoid these mistakes

1
Saying that phagocytes make antibodies. They engulf pathogens; B cells (plasma cells) make antibodies.
2
Describing inflammation as the same as infection. It is a response of the body.
3
Writing that interferon kills viruses. It makes other cells resistant.
4
Confusing neutrophils and macrophages
  • neutrophils are short-lived first responders;
    • macrophages are long-lived and present antigens.

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