I. THE INNATE IMMUNE SYSTEM

D. EARLY INDUCED INNATE IMMUNITY

9. FEVER

 

Fundamental Statements for this Learning Object:

1. Activated macrophages and other leukocytes release inflammatory cytokines such as TNF-alpha, IL-1, and IL-6 when their pattern-recognition receptors (PRRs) bind pathogen associated molecular patterns or PAMPs.
2. These cytokines stimulate the anterior hypothalamus of the brain, the part of the brain that regulates body temperature, to produce prostaglandin E2 , which leads to an increase bodily heat production and increased vasoconstriction.
3. Vasoconstriction decreases the loss of heat from the skin and increases body temperature.
4. Fever increases the environmental temperature above the optimum growth temperature for many microorganisms.
5. Fever leads to the production of heat shock proteins that are recognized by some intraepithelial T-lymphocytes resulting in the production of inflammation-promoting cytokines.
6. Fever elevates the temperature of the body increasing the rate of enzyme reactions, and speeding up metabolism within the body including that involved in innate and adaptive immunity as well as tissue repair.

LEARNING OBJECTIVES FOR THIS SECTION

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D. Early Induced Innate Immunity

Early induced innate immunity begins 4 - 96 hours after exposure to an infectious agent and involves the recruitment of defense cells as a result of pathogen-associated molecular patterns or PAMPs (def) binding to pattern-recognition receptors or PRRs (def). These recruited defense cells include:

• phagocytic cells: leukocytes such as neutrophils, eosinophils, and monocytes; tissue phagocytic cells in the tissue such as macrophages (def);
  
• cells that release inflammatory mediators: inflammatory cells in the tissue such as macrophages and mast cells (def); leukocytes such as basophils and eosinophils; and
  
• natural killer cells (NK cells (def)).

Unlike adaptive immunity, innate immunity does not recognize every possible antigen. Instead, it is designed to recognize molecules shared by groups of related microbes that are essential for the survival of those organisms and are not found associated with mammalian cells. These unique microbial molecules are called pathogen-associated molecular patterns or PAMPs (def) and include LPS from the Gram-negative cell wall, peptidoglycan and lipotechoic acids from the Gram-positive cell wall, the sugar mannose (a terminal sugar common in microbial glycolipids and glycoproteins but rare in those of humans), bacterial and viral unmethylated CpG DNA, bacterial flagellin, the amino acid N-formylmethionine found in bacterial proteins, double-stranded and single-stranded RNA from viruses, and glucans from fungal cell walls. In addition, unique molecules displayed on stressed, injured, infected, or transformed human cells also be recognized as a part of innate immunity. These are often referred to as danger-associated molecular patterns or DAMPs.

Most body defense cells have pattern-recognition receptors or PRRs (def) for these common PAMPs (see Fig. 1) enabling an immediate response against the invading microorganism. Pathogen-associated molecular patterns can also be recognized by a series of soluble pattern-recognition receptors in the blood that function as opsonins and initiate the complement pathways. In all, the innate immune system is thought to recognize approximately 10³ of these microbial molecular patterns.

We will now take a closer look at fever.

9. Fever

Activated macrophages and other leukocytes release inflammatory cytokines such as TNF-alpha, IL-1, and IL-6 when their pattern-recognition receptors (PRRs) bind pathogen associated molecular patterns or PAMPs (def) - molecular components associated with microorganisms but not found as a part of eukaryotic cells. These include bacterial molecules such as peptidoglycan, teichoic acids, lipopolysaccharide, mannans, flagellin, and bacterial DNA. There are also pattern-recognition molecules for viral double-stranded RNA (dsRNA) and fungal cell walls components such as lipoteichoic acids, glycolipids, mannans, and zymosan.

 

These cytokines stimulate the anterior hypothalamus of the brain, the part of the brain that regulates body temperature, to produce prostaglandin E2 (def), which leads to an increase bodily heat production and increased vasoconstriction. This, in turn, decreases the loss of heat from the skin and increases body temperature. Up to a certain point, fever is beneficial:

1. Fever increases the environmental temperature above the optimum growth temperature for many microorganisms. If the microorganisms are growing more slowly, the body's defenses have a better chance of removing them all.

2. Fever leads to the production of heat shock proteins that are recognized by some intraepithelial T-lymphocytes (def) called delta gamma T-cells, resulting in the production of inflammation-promoting cytokines.

3. Fever elevates the temperature of the body increasing the rate of enzyme reactions, and speeding up metabolism within the body. An elevation in the rate of metabolism can increase the production and activity of phagocytes, speed up the multiplication of lymphocytes, increase the rate of antibody and cytokine production, increase the rate at which leukocytes are released from the bone marrow into the bloodstream, and speed up tissue repair.

Too high of a body temperature, however, may cause damage by denaturing the body's enzymes. Hyperpyrexia is a fever with an extreme elevation of body temperature greater than or equal to 41.5 °C (106.7 °F). Body temperature this elevated often indicates a serious underlying condition and may lead to potentially hazardous side effects. As a result, hyperpyrexia is considered as a medical emergency.

 

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Gary E. Kaiser, Ph.D.
Professor of Microbiology
The Community College of Baltimore County, Catonsville Campus
This work is licensed under a Creative Commons Attribution 4.0 International License.
Based on a work The Grapes of Staph at https://cwoer.ccbcmd.edu/science/microbiology/index_gos.html.