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Defenses Against Disease Chapter 36 Pages 691-712.

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Presentation on theme: "Defenses Against Disease Chapter 36 Pages 691-712."— Presentation transcript:

1 Defenses Against Disease Chapter 36 Pages

2 What Causes Disease? Microbes - bacteria, protists, fungi, and viruses Most live in water or the soil most that live in animal bodies but do not harm them and may be beneficial When they cause disease, they are pathogens Most, such as cholera, measles, plague, tetanus, and chicken pox, have been with humans for thousands of years New, more deadly strains of familiar pathogens are called emerging infectious diseases

3 Emerging Infectious Diseases Since the early 1980s - HIV, Ebola virus, West Nile virus, SARS, swine flu, bird flu One strain of the common intestinal bacterium E. coli, which is normally harmless, can cause food poisoning Some Staphylococcus bacteria that normally cannot penetrate the skin cause severe infections or fatal toxic shock syndrome when they enter the body

4 Defense Against Disease Three lines of defense Nonspecific, external barriers Nonspecific, internal defenses Specific internal defenses

5 Nonspecific, External Barriers Pre vent microbes from entering the body Anatomical – skin, cilia, and secretions as tears, saliva, and mucus Cover external surfaces and line the body cavities that come in contact with the external environment Surfaces of the respiratory, digestive, and urogenital tracts

6 Nonspecific, Internal Defenses If the external barriers are breached, the innate immune response, swings into action Components include: White blood cells engulf foreign particles or destroy infected cells Chemicals released by damaged cells and proteins released by white blood cells trigger inflammation and fever These responses operate regardless of the exact nature of the invader, neutralizing the threat

7 Specific, Internal Defenses The final line of defense is the adaptive immune response Immune cells selectively destroy specific invading microbes or toxins and remember the invader This allows for a rapid response to the invader if it reappears in the future



10 Invertebrate Animals Possess only the first two lines of defense Lack adaptive immune responses and must rely on the two nonspecific defenses: External skeletons Slimy secretions White blood cells that attack pathogens and secrete proteins to neutralize the invaders or toxins Defensive proteins, such as lysosome, are similar in vertebrates and invertebrates, suggesting a common ancestor among most of todays animal species

11 How Nonspecific Defenses Function The skin and mucous membranes form external barriers The first line of defense consists of two surfaces with direct exposure to the environment The skin The mucous membranes of the digestive, respiratory, and urogenital tracts

12 Skin The skin and its secretions block entry and provide an inhospitable environment for microbial growth The outer surface of the skin is dry, dead cells filled with tough proteins that prevent microbes from obtaining the water and nutrients they need The secretions from sweat and sebaceous glands contain natural antibiotics, such as lactic acid, that inhibit the growth of many bacteria and fungi

13 Mucous Membranes Antimicrobial secretions, mucus, and ciliary action defend the mucous membranes against microbes Mucous membrane secretions traps microbes They contain antibacterial proteins lysozyme, which kills bacteria by digesting their cell walls defensin, which makes holes in bacterial plasma membranes Cilia on the membranes sweep up the mucus, so it is swallowed or coughed or sneezed out of the body

14 Mucous Membranes

15 More Nonspecific Defenses Stomach – protein digesting enzymes and acidity is lethal The intestines contain harmless bacteria that secrete substances that destroy invading bacteria or fungi Urinary tract - slight acidity of urine inhibits bacterial growth In females, acidic secretions and mucus help protect the vagina Tears, urination, diarrhea, and vomiting all help to expel invaders Despite these defenses, many disease-causing microbes enter the body through the mucous membranes or through cuts in the skin

16 Innate Immune Response Combats invading microbes When microbes penetrate the skin or mucous membranes, they encounter an array of internal defenses, collectively called innate immunity Innate immune responses are nonspecificthat is, they attack many different types of microbes rather than targeting particular invaders

17 Three Categories of Innate Immune Response White blood cells or leukocytes, attack and destroy invading cells or the bodys own cells if they have been infected by viruses The inflammatory response attracts leukocytes to the site of a wound and walls off the injured area, isolating the infected tissue from the rest of the body Fever is produced when microbes start an infection in the body, which both slows down microbial reproduction and enhances the bodys own fighting abilities

18 White Blood Cells Phagocytic leukocytes and natural killer cells destroy invading microbes Several types of leukocytes or phagocytes ingest foreign invaders and cellular debris Three important types of phagocytes are: Macrophages Neutrophils Dendritic cells These cells travel within the bloodstream, move through capillary walls, and patrol the bodys tissues

19 The Attack of the Macrophages

20 Natural Killer Cells Strike at the bodys cells that have become cancerous or been invaded by viruses The surfaces of normal body cells display proteins of the major histocompatibility complex (MHC) which identify the cell as self Natural killer cells kill any nonself cells they encounter by releasing proteins that make holes in the infected or cancerous cells membranes, then secrete enzymes through the holes to kill it

21 Inflammatory Response The inflammatory response attracts phagocytes to injured or infected tissue Tissues become warm, red, swollen, and painful Several functions: It attracts phagocytes to infected or injured tissue It promotes blood clotting It initiates protective behavior by causing pain

22 Inflammatory Response and Mast Cells Inflammatory response begins when damaged cells release chemicals that cause mast cells, to release histamine Histamine relaxes the smooth muscle surrounding arterioles, increasing blood flow and causing capillary walls to become leaky Extra blood flowing through leaky capillaries drives fluid from the blood and into the wounded area, causing redness, warmth, and swelling

23 Chemicals released by wounded cells, mast cells, and by the microbes themselves attract macrophages, neutrophils, and dendritic cells Consume bacteria, dirt, and cellular debris Pus, a thick, whitish mixture of dead bacteria, tissue debris, and white blood cells, may accumulate Other chemicals released by injured cells initiate blood clotting to reduce blood loss and prevent more microbes from entering the blood stream

24 Animation: The Inflammatory Response

25 The Inflammatory Response Phagocytes leave the capillaries and ingest bacteria and dead cells 5 Tissue damage carries bacteria into the wound 1 Wounded cells release chemicals (red) that stimulate mast cells 2 Mast cells release histamine (blue) 3 Histamine increases capillary blood flow and permeability 4 dead cell layer epidermis dermis

26 Fever Combats large-scale infections If invaders breach defenses and mount an infection, they may trigger a fever, which is an important part of the bodys defense against infection The human thermostat, in the hypothalamus of the brain, is set at 97–99ºF During an infection, macrophages release endogenous pyrogen that travels to the hypothalamus and raises the thermostats set point Elevated body temperature increases phagocytic activity and slows bacterial reproduction

27 More Fever Fever also stimulates cells infected by viruses to produce interferon Travels to other cells and increases their resistance to viral attack; also stimulates natural killer cells In an experiment, volunteers were infected with a virus and given aspirin or a placebo Those with aspirin had more viruses in their noses and coughed out more viruses than the controls because fevers in the controls helped reduce the viral infection

28 Adaptive Immune System When nonspecific mechanisms are breached, the body mounts a specific and coordinated adaptive immune response directed against the specific organism The adaptive immune response attacks one specific type of microbe, overcomes it, and provides future protection against only that microbe

29 Components of Adaptive Immune Response Cells of the adaptive immune system are distributed throughout the body, with concentrations of cells in certain locations It consists of three major components: immune cells, tissues and organs, and secreted proteins

30 Immune Cells Adaptive immune response is produced by interactions among several white blood cell types Macrophages, dendritic cells, lymphocytes The key cellular players are B cells and T cells, which arise from stem cells in the bone marrow Some of stem cells complete their development in the bone marrow, becoming B (for bone) cells Others migrate from the marrow to the thymus, where they develop into T (for thymus) cells

31 Tissues and Organs The cells of the immune system are produced and reside in a variety of locations, including: vessels of the lymphatic system the lymph nodes the thymus the spleen patches of specialized connective tissue such as tonsils

32 Immune System Organs Lymph flows through the lymph nodes which contain macrophages and lymphocytes The thymus is essential for development of some immune cells The spleen filters blood, exposing it to white blood cells The tonsils contain macrophages and other white blood cells that sample microbes entering the body through the mouth, destroying many and starting an adaptive immune response

33 thymus spleen bone marrow thoracic duct valve prevents backflow lymph node chambers packed with white blood cells lymph vessels lymph nodes The Lymphatic System Contains Much of the Immune System

34 Secreted Proteins Leukocytes secrete many different proteins, collectively called cytokines, used for communication between cells A number of proteins in the blood, called complement, assist the immune system in killing invading microbes Some cytokines and complement proteins are involved in both the innate and adaptive responses B cells, a type of leukocyte, produce antibodies that help the immune system recognize and destroy invading microbes

35 Adaptive Immune Response Steps All adaptive immune responses include the same three steps: 1. Lymphocytes recognize an invading microbe and distinguish the invader from self 2. They launch an attack 3. They retain a memory of the invader that allows them to ward off future infections by the same type of microbe

36 Moving Along….. To understand how the immune system recognizes invaders and initiates a response, we must answer three questions: How do lymphocytes recognize foreign cells and molecules? How can lymphocytes produce specific responses to so many different types of cells and molecules? How do they avoid mistaking the bodys own cells and molecules for invaders?

37 Recognition of Invaders The adaptive immune system recognizes invaders MHC Bacteria and humans differ because each contains specific, unique, complex molecules Large, complex molecules are antigens, because they are antibody generating molecules that provoke an immune response

38 How Does the Adaptive Immune System Recognize Invaders? Antigens are located on the surface of microbes Sometimes viral antigens become incorporated into plasma membranes of infected cells Viral or bacterial antigens are also displayed on the plasma membranes of dendritic cells and macrophages that engulf them Other antigens, such as toxins released by bacteria, may be in the blood plasma, lymph, or other extracellular fluids

39 Antibody and T-cell Proteins Recognize and bind to foreign antigens Lymphocytes generate two types of proteins that recognize, bind, and help to destroy specific antigens: Antibody proteins, produced by B cells and their offspring T-cell receptor proteins, produced by T cells

40 Antibody Structure Proteins composed of two pairs of peptide chains: one pair of identical large (heavy) chains and one pair of identical small (light) chains Both chains have a constant region, which is similar in all antibodies of the same type, and a variable region that differs among individual antibodies

41 Antibody Binding Sites are 3D The variable regions at the arm tips bind to antigens Each binding site has a unique size, shape and charge so that specific molecules fit in and bind to the antibody The sites are so specific that each antibody can bind only a few, very similar, antigen molecules

42 light chain heavy chain antigen Variable regions form antigen binding sites Constant regions are the same in all antibodies of a given type antigen Antibody Structure

43 Function as Receptors or Effectors Antibodies function as receptors, binding to specific antigens and eliciting a response OR as effectors, helping them destroy cells or molecules that bear the antigen

44 How does it work? As a receptor, the stem anchors the antibody in the plasma membrane of the B cell that produced it, and its two arms stick out from the B cell, sampling the blood and lymph for antigens When the arm of the antibody encounters an antigen with a compatible chemical structure, it binds to it and initiates a response in the B cell As effectors, antibodies are secreted into the bloodstream, where they neutralize antigens, destroy microbes that bear antigens, or attract macrophages that engulf the microbes

45 Antibodies Serve as Receptors or Effectors (a) Antibody receptor function B cell antibody antigen (b) Antibody effector function antibody antigen macrophage microbe

46 Activated T-cells Trigger an Immune Response T-cell receptors recognize invaders and help trigger an immune response T-cell receptors are found on T-cell surfaces Like antibodies, they consist of peptide chains that form specific antigen binding sites Unlike antibodies, T-cell receptors are not released into the bloodstream, and do not directly contribute to the destruction of microbes or toxins T-cell receptors trigger a response in its T cell when the receptor binds an antigen on a cell that has ingested an invading microbe

47 There are millions of them The immune system recognizes millions of different antigens The adaptive immune system recognizes and responds to all antigens that are encountered, because B and T cells produce millions of different antibodies and T-cell receptors How can the body produce so many?

48 Antibody genes are assembled from DNA segments There are not genes for whole antibodies B cells have genes that code for parts constant regions (C), variable regions (V), and regions that connect the two The constant region in each chain is the same for any antibody of a particular type Humans have 200 genes for the variable region of heavy chains and 150 genes for the variable region of the light chain

49 Antibody are Built in B-cells As each B cell develops, it cuts and discards all but one gene of each type, then assembles two unique antibody genes from the genes A heavy-chain gene - consists of one variable and one constant region A light-chain gene - consists of one variable and one constant region Antibodies are produced from composite genes

50 V1V1 V2V2 V3V3 V4V4 V 200 D1D1 D2D2 D 50 J1J1 J2J2 J6J6 CMCM CDCD CGCG CECE CACA V1V1 V2V2 V3V3 V4V4 (a) Genes for parts of the heavy chain (top) and light chain (bottom) of antibodies (b) Complete antibody genes in three different B cells (c) Antibodies synthesized by these three B cells heavy chain light chain heavy chain light chain V 150 J1J1 J2J2 J3J3 J4J4 J5J5 CKCK V2V2 V2V2 D 11 J4J4 J4J4 V 80 J2J2 J2J2 J2J2 CKCK CKCK CKCK V 101 J5J5 CKCK V6V6 J1J1 CKCK CGCG V 87 D8D8 J1J1 CGCG V 111 D 40 J1J1 CGCG Cell 1 V 87 D8D8 J1J1 V 101 J5J5 J5J5 CKCK CKCK Cell 2 V 111 D 40 J1J1 V6V6 V6V6 J1J1 J1J1 CKCK CKCK Cell 3 Cell 1Cell 2Cell 3 CGCG CGCG CGCG CGCG CGCG CGCG Recombination Produces Antibody Genes

51 How many? The random assembly of composite antibody genes yields 3 million unique combinations Further diversity arises because only part of each joining region is actually used in any given antibody Immunologists estimate that 15 to 20 billion unique antibodies are possible - 2 X The result is that each B cell produces an antibody that is different from the one produced by every other B cell, except its own daughter cells

52 T-cell Receptor Construction T-cell receptors are made from different genes, but the process is similar There are more parts available for constructing T-cell receptor genes, so there may be as many as a quadrillion different possible T-cell receptors! 10 15

53 Not Designer Made Antibodies and T-cell receptors are not tailor- made for antigens B and T cells do not design antibodies and T-cell receptors to fit invading antigens Instead, the immune system randomly synthesizes millions of different antibodies and T-cell receptors Antigens almost always encounter antibodies or T-cell receptors that will bind them

54 How to distinguish self from non-self The immune system distinguishes self from non-self Surface of body cells have proteins/polysaccharides that are called the major histocompatibility complex (MHC), unique to each person If the cells of the immune system bind to the antigens of the MHC, they undergo apoptosis or programmed cell death The immune system distinguishes self from non-self by retaining only those immune cells that do not respond to the bodys own molecules

55 Regulatory T-cells Not all self-reactive B and T cells are eliminated in this way Regulatory T-cells prevent these self- reacting lymphocytes from attacking the body and causing autoimmune disease A persons MHC proteins act as foreign antigens in other peoples bodies during organ transplant, donor MHCs must be similar

56 Two types of Attacks The adaptive immune system simultaneously launches two types of attack against antigens: Humoral immunity - B cells and antibodies that they secrete into the blood that attack pathogens outside the bodys cells Cell-mediated immunity is produced by a type of T cell called the cytotoxic T cell that attacks infected body cells, killing both the cell and any pathogens inside it

57 Immunity takes time to develop A person may have millions of different antibodies and T- cell receptors, there is only one cell bearing each type of antibody or T-cell receptor The immune system requires time to be effective because cells recognizing the invader must multiply and differentiate It takes 1 or 2 weeks to mount a strong immune response after the first exposure to an invading microbe

58 An Effective Immune Response Takes Time

59 Humoral Immunity Produced by antibodies carried in the blood Each B cell bears unique antibodies on its surface When an infection occurs, the antibodies borne by a few B cells can bind to antigens on the invader Antigen–antibody binding causes B cells to divide rapidly by the process of clonal selection, producing a population of clones of the original cell

60 Clones or Daughter Cells The daughter cells differentiate into: Memory B cells, play an important role in future immunity to the invader Plasma cells, enlarge and produce a huge quantity of specific antibodies which are released into the bloodstream

61 Animation: B Cell Activation and Differentiation

62 Clonal Selection Among B Cells Invading antigens bind to antibodies on one B cell (dark blue) 1 The B cell selected by the antigen multiplies rapidly 2 A large clone of genetically identical B cells is produced 3 These B cells differentiate into plasma cells and memory B cells 4 Plasma cells release antibodies into the blood 5 endoplasmic reticulum memory B cell plasma cell antibodies antigens antibodies

63 Action of Humoral Antibodies Multiple modes of action- Antibodies in the blood combat invading molecules or microbes in three ways: 1. The circulating antibodies bind to a foreign molecule, virus, or cell and render it harmless by neutralization Example - an antibody covering the active site of a toxic enzyme in snake venom

64 Antibodies block the active site of the toxic enzymes in snake venom snake venom enzyme active site antibody Antibodies Neutralize Toxins

65 2. Antibodies coat the surface of invading molecules, viruses, or cells and make it easier for phagocytic cells to destroy them Macrophages recognize the antibody stems sticking out into the blood, then engulf the antibody-coated invaders and digest them 3. When antibodies bind to antigens on the surface of a microbe, the antibodies interact with complement proteins that are present in the blood Some of the complement proteins punch holes in the plasma membranes of the microbe, killing it Other complement proteins promote phagocytosis

66 Humoral immunity fights invaders that are outside cells Antibodies cannot enter cells; therefore, the humoral response is effective only against antigens when they are outside of cells, in the blood or extracellular fluid Viruses are vulnerable when they are outside body cells, but after they enter a body cell, they are safe from antibody attack Cell-mediated immune reactions are required to fight viruses once they have entered body cells

67 Cell-mediated Immunity Produced by cytotoxic T cells Is the bodys primary defense against cells that are cancerous or that have been infected by viruses Cytotoxic T-cells in the blood bump into an infected body cell that is displaying a viral protein on its surface The cytotoxic T-cell receptor will bind to the viral protein and squirt proteins onto the surface of the infected cell, punching holes in the cell and killing it Cancer cells display unusual proteins that the cytotoxic T cells recognize as foreign, and are killed as a result

68 Cell-mediated Immunity in Action

69 Helper T-cells Helper T cells enhance both humoral and cell-mediated Immunity B-cells and cytotoxic T-cells are not effective without assistance from helper T cells Helper T cells have receptors that bind to antigens displayed on the surfaces of dendritic cells or macrophages that have engulfed and digested invading microbes When its receptor binds an antigen, a helper T cell multiplies rapidly, and its daughter cells differentiate and release cytokinins that stimulate cell division and differentiation in both B cells and cytotoxic T cells

70 Both B-cells and cytotoxic T-cells are most effective against infection when they receive stimulation by cytokinins from helper T cells Human immunodeficiency virus (HIV) kills helper T cells Without these cells, the immune system cannot fight off diseases that would otherwise be trivial

71 A Summary of Humoral and Cell-Mediated Immune Response B cell helper T cell cytotoxic T cell cytokines infected cell dendritic cell or macrophage antibody viral antigen Targets invaders outside cells (e.g., viruses, bacteria, fungi, protists, and toxins) Stimulate both humoral and cell-mediated immunity by releasing cytokines Targets defective body cells (e.g., infected cells and cancer cells), transplants HUMORAL IMMUNITYCELL-MEDIATED IMMUNITYHELPER T CELLS B-cell antibodies bind to viral antigens and stimulate the B cells to divide and differentiate Viral antigens presented on the surfaces of dendritic cells or macrophages, and infected cells T-cell receptors bind to viral antigens Cytokines released by helper T cells stimulate B cells and cytotoxic T cells virus

72 A Summary of Humoral and Cell- Mediated Immune Responses memory cytotoxic T cell memory helper T cell memory B cell infected cell cytotoxic T cell plasma cell Plasma cells secrete antibodies into the blood and extracellular fluid Memory cells confer future immunity to this virus Cytotoxic T cells release pore-forming proteins that destroy infected cells B cell helper T cell cytotoxic T cell cytokines antibody Targets invaders outside cells (e.g., viruses, bacteria, fungi, protists, and toxins) Stimulate both humoral and cell-mediated immunity by releasing cytokines Targets defective body cells (e.g., infected cells and cancer cells), transplants HUMORAL IMMUNITYCELL-MEDIATED IMMUNITYHELPER T CELLS Cytokines released by helper T cells stimulate B cells and cytotoxic T cells


74 How Does the Adaptive Immune System Remember? After recovering from a disease, you remain immune to that particular microbe, perhaps a lifetime Some of the daughter cells of the original B cells, cytotoxic T cells, and helper T cells that responded to the original infection differentiate into memory B cells and memory T cells and survive for many years If the body is reinvaded by the same type of microbe, the memory cells recognize the invader and mount an immune response

75 Animation: Humoral Versus Cell-Mediated Immunity

76 Memory Cells Memory B cells rapidly produce a clone of plasma cells, secreting antibodies that combat this second invasion Memory T cells produce clones of either helper T cells or cytotoxic T cells specific for the remembered invader Each memory cell responds so fast and so largely in a second infection, the body fends off the attack before the person suffers any symptomsthey have become immune Acquired immunity confers long-lasting protection against many diseases such as small pox, measles, mumps, and chicken pox

77 Animation: Memory B Cells

78 Acquired Immunity

79 Drug Therapy Antibiotics slow down microbial reproduction Antibiotics - drugs that combat infection by slowing down the multiplication of bacteria The occasional mutant microbe that is resistant to an antibiotic will pass its genes for resistance to its offspring, which results in many antibiotics becoming ineffective Antibiotics are not effective against viruses Drugs are available that target different stages of the viral cycle of infection, and are used to treat HIV, severe herpes, and in some cases, the flu virus

80 Vaccines and Immunity Vaccinations stimulate the development of memory cells and future immunity against disease A vaccine stimulates an immune response by exposing a person to a pathogens antigens May consist of weakened or killed microbes, or some of the pathogens antigens Exposure to these antigens results in the body producing memory cells that confer immunity against similar microbes

81 Allergies Misdirected immune response Allergies are immune reactions to harmless substances that are treated (by the body) as if they were pathogens Common allergies include pollen, mold spores, bee or wasp venoms, and some foods such as milk, eggs, fish, wheat, tree nuts, and peanuts

82 Immune System Malfunctions All allergic reaction begins when allergy-causing antigens, called allergens, enter the body and bind to allergy antibodies on a special type of B cell This B cell proliferates, producing plasma cells that pour out allergy antibodies into the plasma The antibodies attach to mast cells, mostly in the respiratory and digestive tracts

83 Allergies If allergens bind to these attached antibodies, they trigger the release of histamine, which causes leaky capillaries and other symptoms of inflammation In the respiratory tract, histamine increases mucous secretions and results in symptoms Food allergies may cause intestinal cramps and diarrhea; some reactions are so strong that the airways may completely close, causing death by suffocation

84 An Allergic Reaction to Pollen Plasma cells produce allergy antibodies 2 First exposure to pollen (yellow) stimulates B cells to produce allergy plasma cells 1 Allergy antibodies bind to mast cells 3 Reexposure to pollen results in pollen binding to allergy antibodies on mast cells 4 plasma cell mast cell Binding of pollen stimulates mast cells to release histamine (blue), triggering the inflammatory response 5

85 Autoimmune Disease An immune response against the bodys own molecules Occasionally, our immune system produces anti-self antibodies The result is an autoimmune disease in which the immune system attacks a component of ones own body, such as a type of anemia where antibodies destroy a persons red blood cells Type 1 diabetes begins when the immune system attacks insulin-secreting cells of the pancreas Other autoimmune diseases include rheumatoid arthritis, multiple sclerosis, and systemic lupus

86 Therapy for Autoimmune Disease No known cures Replacement therapy can alleviate the symptoms for example, by giving insulin to diabetics or blood transfusions to anemia victims The autoimmune response can be reduced with drugs that suppress the immune response This course of action also reduces responses to the everyday assaults of disease microbes, so the therapy has drawbacks

87 Immune Deficiency Diseases Occur when the body cannot mount an effective immune response There are two disorders in which the immune system cannot combat routine infections: Severe combined immune deficiency (SCID), a group of genetic defects in which few or no immune cells are formed Acquired immune deficiency syndrome (AIDS), where a viral infection destroys a formerly functional immune system

88 HIV Causes AIDS

89 SCID Severe combined immune deficiency is inherited A child with severe combined immune deficiency (SCID) may survive the first few months of postnatal life, protected by antibodies acquired from the mother during pregnancy Once these antibodies are lost, common infections can prove fatal because the child lacking an immune system cannot generate an effective immune response A form of therapy is to transplant bone marrow from a healthy donor into the child to provide enough immune cells to confer normal immune responses

90 AIDS Acquired immune deficiency disease The most common immune deficiency disease AIDS is caused by human immunodeficiency viruses (HIV) that infect and destroy helper T cells, stimulating both the cell-mediated and humoral immune responses AIDS does not kill people directly, but victims become increasingly susceptible to other diseases as their helper T-cell populations decline

91 How HIV is spread Because HIV cannot survive for long outside the body, it is transmitted only by the direct contact of broken skin or mucous membranes with virus-laden body fluids, including blood, semen, vaginal secretions, and breast milk HIV is spread by sexual activity, sharing needles among intravenous drug users, or through blood transfusions HIV enters a helper T cell and hijacks the cells metabolic machinery, forcing it to make more viruses which then emerge, taking an outer coating of T-cell membrane with them

92 Early in the infection, as the immune system fights the virus, the victim may develop a fever, rash, muscle aches, headaches, and enlarged lymph nodes Over time helper T cell levels drop, severely weakening immune response As HIV levels increase, they kill more helper T cells and the person is more succeptible to other infections

93 Animation: HIV- The AIDS Virus

94 Several drugs can slow down the replication of HIV and thereby slow the progress of AIDS; unfortunately, HIV can mutate into forms that are resistant to the drugs Some HIV-positive individuals receiving the best medical care often live out a normal life span The best solution would be to develop a vaccine This is a challenge, HIV disables the immune response that a vaccine depends on HIV has a high mutation rate, perhaps a 1000X times faster than flu viruses Lone infected individuals may harbor different strains of HIV in their blood and semen because of mutations that occurred within their bodies

95 Immunity and Cancer Cancer will kill more than 500,000 people in the United States this year Approximately 40% of U.S. citizens will eventually contract some form of cancer Triggered by environmental factors such as UV radiation, smoking, faulty genes, mistakes during cell division, and viruses These triggers produce cancer by sabotaging the mechanisms that normally control the growth of the bodys own cells

96 Immune System recognizes most cancer cells as foreign Cancer cells are self and the immune response usually does not respond to self The process that causes a cell to become cancerous leads to slightly different proteins appearing on their surfaces Natural killer cells and cytotoxic T cells encounter these new proteins, recognize them as non-self antigens, and destroy the cancer cells Some cancer cells do not bear antigens that allow the immune system to recognize them as foreign or, as in leukemia, suppress the immune system

97 Vaccination can prevent some cancers Some cancers are caused by viruses - including some cancers of the liver, mouth, throat, penis; some types of leukemia; and cervical cancers Two vaccines are available that help prevent certain cancers: Hepatitis B, which reduces the risk of liver cancer Human papilloma virus, which cause most cases of cervical cancer

98 Some treatment vaccines can cure certain cancers by providing a patient with antigens commonly found on cells of the type of cancer that the patient has, often enhanced in various ways to boost the patients immune response against the cancer Current trials of this vaccine against prostate cancer and melanoma are in progress Other treatment vaccines consist of antigens from a patients own tumor cells, often enhanced to stimulate a stronger immune response

99 Another approach is to take antigen-presenting dendritic cells from a patient, expose them to antigens from cancer cells, and force them to multiply rapidly in culture The resulting daughter cells are then injected back into the patient This large number of activated dendritic cells should stimulate the patients own anticancer immune response

100 Medical Treatments for Cancer Most depend on selectively killing cancerous cells Attempts to eliminate cancer mostly focus on surgery, radiation, and chemotherapy Surgically removing the tumor is the first step in treating many cancers, but it can be difficult to remove all the cancerous tissue Tumors can be treated with radiation, which can destroy even microscopic clusters of cancer cells by disrupting their DNA, preventing cell division Neither surgery nor radiation is effective against cancer that has spread throughout the body

101 Chemotherapy Drugs Commonly used to supplement surgery or radiation Attack the machinery of cell division, so they are somewhat selective for cancer cells, which divide more faster than normal cells Chemotherapy also kills some healthy, dividing cells Damage to dividing cells in patients hair follicles and intestinal lining by chemotherapy produces its well-known side effects of hair loss, nausea, and vomiting

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