Chapt 43 - CHAPTER 43 THE BODYS DEFENSES Introduction An...

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CHAPTER 43 THE BODY’S DEFENSES Introduction An animal must defend itself against unwelcome intruders—the many potentially dangerous viruses, bacteria, and other pathogens it encounters in the air, in food, and in water. It must also deal with abnormal body cells, which, in some cases, may develop into cancer. Three cooperative lines of defense have evolved to counter these threats. Two of these are nonspecific—that is, they do not distinguish one infectious agent from another. The first line of nonspecific defense is external, consisting of epithelial cells that cover and line our bodies and the secretions they produce. The second line of nonspecific defense is internal, involving phagocytic cells and antimicrobial proteins that indiscriminately attack invaders that penetrate the body’s outer barriers. The third line of defense, the immune system, responds in a specific way to particular toxins, microorganisms, aberrant body cells, and other substances marked by foreign molecules. Specific defensive proteins called antibodies are produced by lymphocytes. A. Nonspecific Defenses Against Infection An invading microbe must penetrate the external barrier formed by the skin and mucous membranes, which cover the surface and line the openings of an animal’s body. If it succeeds, the pathogen encounters the second line of nonspecific defense, interacting mechanisms that include phagocytosis, the inflammatory response, and antimicrobial proteins. 1. The skin and mucous membrane provide first-line barriers to infection Intact skin is a barrier that cannot normally be penetrated by bacteria or viruses, although even minute abrasions may allow their passage. Likewise, the mucous membranes that line the digestive, respiratory, and genitourinary tracts bar the entry of potentially harmful microbes. Beyond their role as a physical barrier, the skin and mucous membranes counter pathogens with chemical defenses. In humans, for example, secretions from sebaceous and sweat glands give the skin a pH ranging from 3 to 5, which is acidic enough to prevent colonization by many microbes. Microbial colonization is also inhibited by the washing action of saliva, tears, and mucous secretions that continually bathe the exposed epithelium. All these secretions contain antimicrobial proteins. One of these, the enzyme lysozyme , digests the cell walls of many bacteria, destroying them. Mucus , the viscous fluid secreted by cells of mucous membranes, also traps microbes and other particles that contact it. In the trachea, ciliated epithelial cells sweep out mucus with its trapped microbes, preventing them from entering the lungs. Microbes present in food or water, or those in swallowed mucus, must contend with the highly acidic environment of
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This note was uploaded on 12/14/2009 for the course BIOCHEM bIO taught by Professor Professor during the Spring '09 term at École Normale Supérieure.

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Chapt 43 - CHAPTER 43 THE BODYS DEFENSES Introduction An...

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