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Human Biology (Year 12) - Defence Against Pathogens

Ben Whitten

What is fever?

Fever is a homeostatic mechanism and an immune response to infection which results in the human body's core temperature rising to a level above its set point. The body's "thermostat" is reset to a higher temperature, and as a result the body feels cold and responds by trying to further increase body temperature through;

  • Shivering to generate heat (rapid contraction of muscles)

  • Vasoconstriction of blood vessels (reducing heat loss by reducing diameter of blood vessels)

How is a fever established?
  1. Macrophages respond to the entry of a pathogen (foreign body) by ingesting it, through phagocytosis

  2. The macrophage then begins secreting chemicals such as Interleukin 1, a chemical called a pyrogen (some pathogens also release other chemicals which are pyrogens)

  3. Other chemicals released from immune cells termed Cytokines result in the production of pyrogens

  4. Pyrogens are released into the blood (pyrogens are chemicals which travel through the blood and act by altering the body's temperature sensors/"thermostat", i.e. making the body feel cooler than it is)

  5. The hypothalamus responds to the pyrogens by initiating thermoregulatory mechanisms to raise body temperature

What are the benefits of a fever?

Although increases in body temperature can be dangerous if they are too high (temperatures above 44.5-45 degrees Celsius results in death as biochemical processes are disrupted), there are a variety of outcomes which are useful in the defence against a pathogen in infection.

  • High core temperature inhibits actions of the pathogen or kills it

  • Effects of interferon are amplified, an immune chemical which interferes with viral replication

  • Increases killer T-cell production to speed up the cell mediated response

  • Increases metabolic rate to amplify damaged tissue repair

  • Increases heart rate and blood flow, allowing lymphocytes to reach infection sites more rapidly

What happens after a fever?

When a fever breaks, this is referred to as the crisis point and the body temperature is reset to normal. The body now feels too hot, and thermoregulatory mechanisms act to reduce the core body temperature back to approximately 37 degrees Celsius.Vasodilation of blood vessels in the skinIncreased sweat production

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