* AP IMMUNE SYSTEM
* 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. * Herpes simplex-1 (Herpes labialis, HSV-1)
* Herpes simplex 2 (HSV-2, Herpes genitalis) on thigh
* 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. * 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 the stomach. * The acid destroys many microbes before they can enter the intestinal tract. * One exception, the virus hepatitis A, can survive gastric acidity and gain access to the body via the digestive tract. * 2. Phagocytic cells, inflammation, and antimicrobial proteins function early in infection * Microbes that penetrate the first line of defense face the second line of defense, which depends mainly on phagocytosis, the ingestion of invading organisms by certain types of white cells. * Phagocyte function is intimately associated with an effective inflammatory response and also with certain antimicrobial proteins. * The phagocytic cells called neutrophils constitute about 60%-70% of all white blood cells (leukocytes). * Cells damaged by invading microbes release chemical signals that attract neutrophils from the blood. * The neutrophils enter the infected tissue, engulfing and destroying microbes there. * Neutrophils tend to self-destruct as they destroy foreign invaders, and their average life span is only a few days. * Monocytes, about 5% of leukocytes, provide an even...
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