How does the body defend itself against pathogens without targeting a specific one?
The non-specific (innate) defences of the body, including physical and chemical barriers, the inflammatory response (the release of histamine, vasodilation and increased capillary permeability), and the action of phagocytes by phagocytosis.
An SQA Higher Human Biology answer on the non-specific body defences, covering the physical and chemical barriers to pathogens, the inflammatory response including histamine, vasodilation and increased capillary permeability, the action of phagocytes by phagocytosis, and the release of cytokines.
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What this dot point is asking
The SQA wants you to describe the physical and chemical barriers that keep pathogens out, explain the inflammatory response (the role of histamine, vasodilation and increased capillary permeability), and describe how phagocytes destroy pathogens by phagocytosis and release cytokines.
Physical and chemical barriers
The first barrier is physical and chemical:
- Physical barriers keep pathogens out. The skin forms a tough outer layer, and mucous membranes lining the airways and gut trap pathogens in sticky mucus, which is then moved away or swallowed.
- Chemical barriers destroy pathogens that land on body surfaces. Examples include the acid in the stomach, which kills many swallowed bacteria, and the enzyme lysozyme in tears and saliva, which breaks down bacterial cell walls.
These barriers stop most pathogens before they ever enter the tissues.
The inflammatory response
If a pathogen breaks through the barriers, for example through a cut, the inflammatory response is triggered:
The increased blood flow brings phagocytes, clotting factors and (later) antibodies to the infected area. The familiar signs of inflammation, redness, heat, swelling and pain, are all the result of histamine widening the vessels and making them leaky.
Phagocytosis
The phagocyte recognises the pathogen and engulfs it, taking it into a vacuole inside the cell. Lysosomes, which contain digestive enzymes, then fuse with the vacuole and digest and destroy the pathogen. Phagocytes also release chemical signals called cytokines, which attract more phagocytes to the site and help coordinate the wider immune response, including activating the specific immune system. This is the bridge between the non-specific and specific defences.
Examples in context
Example 1. Lysozyme in tears. Tears continually wash the surface of the eye and contain the enzyme lysozyme, which breaks down the cell walls of bacteria. This chemical barrier helps protect the exposed eye from infection without targeting a specific pathogen.
Example 2. Cytokines raising the alarm. When phagocytes engulf bacteria at a wound, they release cytokines that draw still more phagocytes to the site and switch on the specific immune response. This shows how the non-specific defences alert and recruit the rest of the immune system.
Try this
Q1. Name the chemical released by damaged tissue that triggers vasodilation in inflammation. [1 mark]
- Cue. Histamine.
Q2. Explain how lysosomes help a phagocyte destroy an engulfed pathogen. [1 mark]
- Cue. Lysosomes fuse with the vacuole and release digestive enzymes that break down and destroy the pathogen.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20194 marksDescribe the inflammatory response that occurs at the site of an infected wound.Show worked answer →
A 4-mark answer needs the chemical signal and the changes it causes.
When tissue is damaged or infected, certain cells release the chemical histamine. Histamine causes the small blood vessels at the site to widen, a process called vasodilation, which increases blood flow to the area and makes it red and warm.
Histamine also increases the permeability of the capillaries, so more fluid, and white blood cells called phagocytes, leak out into the tissue. This causes the swelling and brings phagocytes to the site to destroy the pathogens. The increased blood flow also delivers clotting factors and antibodies.
Award (1) histamine is released from damaged tissue, (2) vasodilation increases blood flow (redness and heat), (3) increased capillary permeability causes swelling, and (4) phagocytes are delivered to the site to fight infection.
SQA Higher 20213 marksDescribe the process of phagocytosis and state the role of cytokines released by phagocytes.Show worked answer →
This is a 3-mark question on phagocytes.
In phagocytosis, a phagocyte (a type of white blood cell) recognises and engulfs a pathogen, taking it into a vacuole inside the cell. Enzymes contained in lysosomes then fuse with the vacuole and digest and destroy the pathogen.
Phagocytes also release chemical signalling molecules called cytokines. Cytokines attract more phagocytes to the site of infection and help to coordinate the immune response, including activating the specific immune system.
Award (1) the phagocyte engulfs the pathogen into a vacuole, (2) lysosome enzymes digest and destroy it, and (3) cytokines attract more phagocytes and coordinate the response.
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