9.2.4 Temperature Response

Fever or pyrexia is the raised body temperature than normal. The invaded microorganisms often release certain chemicals, which are generally termed as pyrogens. These pyrogens cause the temperature set point of the hypothalamic thermostat of the body to rise. As a result, all the mechanisms for raising the body temperature are brought into play, such as heat conservation and increased heat production.

Definition

Fever (Pyrexia): Fever or pyrexia is the raised body temperature than normal.

Pyrogens: The invaded microorganisms often release certain chemicals, which are generally termed as pyrogens. These pyrogens cause the temperature set point of the hypothalamic thermostat of the body to rise.

Endogenous Pyrogens: Certain white blood cells in response to the infection also release hormones collectively called endogenous pyrogens that further increase the temperature set point of hypothalamus.

Main Idea

• Fever or pyrexia is raised body temperature than normal.
• Invaded microorganisms release certain chemicals called pyrogens.
• Pyrogens cause the temperature set point of the hypothalamic thermostat to rise.
• All mechanisms for raising body temperature are brought into play:
• Heat conservation
• Increased heat production
• Certain white blood cells also release endogenous pyrogens.
• Endogenous pyrogens further increase the temperature set point of hypothalamus.
• Higher body temperature than normal increases the activity of phagocytic white blood cells.
• Endogenous pyrogens also cause other cells to reduce the concentration of iron in the blood.
• Many bacteria require more iron to reproduce at 38°C or 39°C than at 37°C.
• Fever and reduced iron in the blood combine to slow down the rate of reproduction of bacteria.
• Fever also increases the production of interferons.
• Interferons travel to other cells and increase their resistance to viral attack.
• Higher body temperature may directly inactivate virus particles.
• Replication of some viruses is reduced at higher temperatures.
• Fever is nonspecific defence against microbial infections.
• High degree fever can be destructive for body's own metabolic system.
• Physicians prescribe antipyretic drugs to patients of high degree fever.

Core Concept

• Fever = raised body temperature
• Pyrogens = chemicals released by microorganisms
• Endogenous pyrogens = released by white blood cells
• Hypothalamic thermostat = set point rises
• Mechanisms = heat conservation; increased heat production
• Effects of fever:
• Increases activity of phagocytic white blood cells
• Reduces iron concentration in blood
• Increases production of interferons
• Inactivates virus particles
• Reduces replication of some viruses
• Antipyretic drugs = reduce fever

Detailed Explanation

Temperature Response

• Fever or pyrexia is the raised body temperature than normal.
• The invaded microorganisms often release certain chemicals, which are generally termed as pyrogens.
• These pyrogens cause the temperature set point of the hypothalamic thermostat of the body to rise; as a result, all the mechanisms for raising the body temperature are brought into play, such as heat conservation and increased heat production.
• Since, higher body temperature than normal facilitates the microbial growth in the body, this is the reason why invaded microorganisms want to increase the host's body temperature.
• On the other hand, certain white blood cells in response to the infection, also release hormones collectively called endogenous pyrogens that further increase the temperature set point of hypothalamus because higher body temperature than normal increases the activity of phagocytic white blood cells that attack upon bacteria.
• The endogenous pyrogens also cause other cells to reduce the concentration of iron in the blood because many bacteria require more iron to reproduce at temperature of 38°C or 39°C than at 37°C so fever and reduced iron in the blood combine to slow down their rate of reproduction.
• Fever also increases the production of interferons that travel to other cells and increase their resistance to viral attack.
• The higher body temperature may directly inactivate the virus particles, particularly enveloped viruses, which are more heat-sensitive than non-enveloped viruses.
• Replication of some viruses is reduced at higher temperatures, therefore fever may inhibit replication.
• However, fever is nonspecific defence against microbial infections, but often, high degree fever becomes destructive for body's own metabolic system and ultimately body is collapsed.
• Therefore, physicians use to prescribe antipyretic drugs to the patients of high degree fever.

Science, Technology and Society Connections

Justify why the physician prescribe antipyretic drugs, when fever is a nonspecific defence against microbial infections.

- Antipyretic drugs create their effects by inhibiting prostaglandin production in the hypothalamus, which has the effect of blocking set point elevation and maintaining the set point at nearer normal levels.

Summary Table of Temperature Response

Summary of Effects of Fever

Intermediate Level

Temperature Response

Effects of Fever

Antipyretic Drugs

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Fever is raised body temperature.
• 2. Fever is also called pyrexia.
• 3. Invaded microorganisms release pyrogens.
• 4. Pyrogens cause the temperature set point to rise.
• 5. Mechanisms for raising body temperature are brought into play.
• 6. Heat conservation occurs.
• 7. Increased heat production occurs.
• 8. White blood cells release endogenous pyrogens.
• 9. Endogenous pyrogens further increase the temperature set point.
• 10. Higher body temperature increases phagocytic activity.
• 11. Endogenous pyrogens reduce iron concentration in blood.
• 12. Many bacteria require more iron to reproduce at 38°C or 39°C.
• 13. Fever and reduced iron slow down bacterial reproduction.
• 14. Fever increases production of interferons.
• 15. Interferons increase resistance to viral attack.
• 16. Higher body temperature inactivates virus particles.
• 17. Replication of some viruses is reduced at higher temperatures.
• 18. Fever is nonspecific defence against microbial infections.
• 19. High degree fever can be destructive.
• 20. Physicians prescribe antipyretic drugs.

Remember

• - Fever = raised body temperature
• - Pyrogens = chemicals released by microorganisms
• - Endogenous pyrogens = released by white blood cells
• - Hypothalamic thermostat = set point rises
• - Mechanisms = heat conservation; increased heat production
• - Effects of fever:
• - Increases activity of phagocytic white blood cells
• - Reduces iron concentration in blood
• - Increases production of interferons
• - Inactivates virus particles
• - Reduces replication of some viruses
• - Antipyretic drugs = reduce fever

Common Mistakes

• 1. Confusing fever and hyperthermia — Fever is regulated; hyperthermia is unregulated.
• 2. Forgetting the role of pyrogens — Raise the temperature set point.
• 3. Mixing up endogenous and exogenous pyrogens — Endogenous from WBCs; exogenous from microorganisms.
• 4. Forgetting the role of iron — Reduced in blood during fever.
• 5. Mixing up the effects of fever — Increased phagocytic activity; reduced iron; increased interferons.
• 6. Believing fever is always bad — Fever is a nonspecific defence.
• 7. Mixing up the mechanisms — Heat conservation; increased heat production.
• 8. Forgetting the role of antipyretic drugs — Inhibit prostaglandin production.
• 9. Thinking high fever is not dangerous — High fever can be destructive.
• 10. Mixing up the effects on viruses — Inactivation; reduced replication.

Quick Revision

Fever or pyrexia is the raised body temperature than normal. The invaded microorganisms often release certain chemicals, which are generally termed as pyrogens. These pyrogens cause the temperature set point of the hypothalamic thermostat of the body to rise; as a result, all the mechanisms for raising the body temperature are brought into play, such as heat conservation and increased heat production. Since, higher body temperature than normal facilitates the microbial growth in the body, this is the reason why invaded microorganisms want to increase the host's body temperature. On the other hand, certain white blood cells in response to the infection, also release hormones collectively called endogenous pyrogens that further increase the temperature set point of hypothalamus because higher body temperature than normal increases the activity of phagocytic white blood cells that attack upon bacteria. The endogenous pyrogens also cause other cells to reduce the concentration of iron in the blood because many bacteria require more iron to reproduce at temperature of 38°C or 39°C than at 37°C so fever and reduced iron in the blood combine to slow down their rate of reproduction. Fever also increases the production of interferons that travel to other cells and increase their resistance to viral attack. The higher body temperature may directly inactivate the virus particles, particularly enveloped viruses, which are more heat-sensitive than non-enveloped viruses. Replication of some viruses is reduced at higher temperatures, therefore fever may inhibit replication. However, fever is nonspecific defence against microbial infections, but often, high degree fever becomes destructive for body's own metabolic system and ultimately body is collapsed. Therefore, physicians use to prescribe antipyretic drugs to the patients of high degree fever.

60-Second Revision

• Fever = raised body temperature
• Pyrogens = chemicals released by microorganisms
• Endogenous pyrogens = released by white blood cells
• Hypothalamic thermostat = set point rises
• Mechanisms = heat conservation; increased heat production
• Effects of fever:
• Increases activity of phagocytic white blood cells
• Reduces iron concentration in blood
• Increases production of interferons
• Inactivates virus particles
• Reduces replication of some viruses
• Antipyretic drugs = reduce fever
• Warning: High degree fever can be destructive for body's own metabolic system

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• 9. Immunity

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• 9.3 THIRD LINE OF DEFENCE - THE SPECIFIC DEFENCES

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