Chapter 43 The Immune SystemLecture OutlineOverview: Reconnaissance, Recognition, and Response- 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.- Two major kinds of defense have evolved to counter these threats.- The first kind of defense is innate immunity.- Innate defenses are largely nonspecific, responding to abroad range of microbes.- Innate immunity consists of external barriers formed by the skin and mucous membranes, plus a set of internal cellular and chemical defenses that defend against microbes that breach the external barriers.- The internal defenses include macrophages and other phagocytic cells that ingest and destroy pathogens.- A second kind of defense is acquired immunity.- Acquired immunity develops only after exposure to microbes, abnormal body cells, or other foreign substances.- Acquired defenses are highly specific and can distinguish one inducing agent from another.- This recognition is achieved by white blood cells called lymphocytes, which produce two general types of immune responses. In the humoral response, cells derived from B-lymphocytes secrete defensive proteins called antibodies that bind to microbes and target them for elimination. In the cell-mediated response, cytotoxic lymphocytesdirectly destroy infected body cells, cancer cells, or foreign tissue.Concept 43.1 Innate immunity provides broad defenses against infection- An invading microbe must penetrate the external barrier formed by the skin and mucous membranes, whichLecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 43-1cover the surface and line the openings of an animal’s body.- If it succeeds, the pathogen encounters the second line of nonspecific defense, innate cellular and chemical mechanisms that defend against the attacking foreign cell. 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.- Cells of these mucous membranes produce mucus, a viscous fluid that traps microbes and other particles.- In the trachea, ciliated epithelial cells sweep out mucus with its trapped microbes, preventing them from entering the lungs.- 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 cellwalls of many bacteria, destroying them.- 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 enterthe intestinal tract.- One exception, the virus hepatitis A, can survive gastricacidity and gain access to the body via the digestive tract. Phagocytic cells 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.Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 43-2- Phagocyte function is intimately associated with an effective inflammatory response and also with certain antimicrobial proteins.- Phagocytes attach to their prey via surface receptors found on microbes but not normal body cells.- After attaching to the microbe, a phagocyte engulfs it, forming a vacuole that fuses with a lysosome.- Microbes are destroyed within lysosomes in two ways. Lysosomes contain nitric oxide and other toxic forms of oxygen, which act as potent antimicrobial agents. Lysozymes and other enzymes degrade mitochondrial components.- Some microbes have adaptations that allow them to evade destruction by phagocytes.- The outer capsule of some bacterial cells hides their surface polysaccharides and prevents phagocytes from attaching to them.- Other bacteria are engulfed by phagocytes but resist digestion, growing and reproducing within the cells.- Four types of white blood cells are phagocytic.- 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 foreigninvaders, and their average life span is only a few days.- Monocytes, about 5% of leukocytes, provide an even more effective phagocytic defense.- After a few hours in the blood, they migrate into tissuesand develop into macrophages, which are large, long-lived phagocytes.- Some macrophages migrate throughout the body, whileothers reside permanently in certain tissues, including the lungs, liver, kidneys, connective tissues, brain, and especially in lymph nodes and the spleen.- The fixed macrophages in the spleen, lymph nodes, andother lymphatic tissues are particularly well located to contact infectious agents.- Microbes that enter the blood become trapped in the spleen, while microbes in interstitial fluid flow into lymph and are trapped in lymph nodes.- In either location, microbes soon encounter resident macrophages.Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 43-3- Eosinophils, about 1.5% of all leukocytes, contribute to defense against large parasitic invaders, such as the blood fluke, Schistosoma mansoni.- Eosinophils position themselves against the external wall of a parasite and discharge destructive enzymes from cytoplasmic granules.- Dendritic cells can ingest microbes like macrophages. However, their primary role is to stimulate the development of acquired