15-11 Inflammation is a response to tissue damage that can activate the immune system

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• Inflammation occurs in response to tissue damage, either through injury or from infection.
• In the case of an infection, inflammation causes the activation of the immune response.

The next time you burn or cut yourself, make a note of the symptoms you observe in the damaged tissue. You will observe swelling, redness, heat and pain. Swelling is caused by the creation of gaps between the capillary cells, allowing the movement of fluid and immune cells to the damaged area. An increase in blood flow to the area causes the characteristic redness. Heat is caused by the accumulation of blood and the release of fever-inducing molecules (called pyrogens). Pain is felt in response to tissue damage and the irritation of sensory nerves in the affected area. This series of events is collectively called inflammation.

Of all the host defenses, inflammation is arguably the most important. It is one of the initial responses to the presence of an invader and focuses the immune system at the site of infection. Inflammation draws phagocytes, which attack and kill the invading microbe and interact with the adpative immune system to create a long-term response. However, inflammation can cause considerable damage to host tissues, which can be part of the microbial pathogenesis of a disease process. In addition, unwanted inflammation can also be a major destructive force, as seen in many autoimmune diseases.

Inflammation can be triggered by damage to host tissue, by certain immunological reactions (such as the complement cascade discussed previously) or by the presence of harmful microbial as well as non-microbial agents. No matter what the cause, induction and maintenance of inflammation follows a similar series of events as shown in Figure 15-10.

Injured or dead cells release their cytoplasmic constituents in the surrounding area causing a lowering of the pH.

1. The drop in pH leads to the activation of the extracellular enzyme kallikrein that in turn activates bradykinin.
2. Bradykinin binds to receptors in the cells of nearby capillary walls, opening up intracellular spaces that allow blood constituents, including immune cells, access to the area.
3. Bradykinin also binds to mast cells that are found in association with the small vessels of most tissues. This binding precipitates the release of the mediators of inflammation such as histamine, heparin, prostaglandins, leukotriene and other compounds.
4. This cocktail of chemicals recruits neutrophils, macrophages and leukocytes that destroy the invading particles and help to induce a specific immune response.

One of the major results of inflammation is the isolation of the pathogen in a defined area. This often occurs through the formation of a blood clot--the same kind of clot that forms when the skin is cut. This response is a rapid and extremely powerful antimicrobial activity that successfully deals with many infections.

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