Chapter 43

1. Chapter 43 The Immune System

2. Overview: Reconnaissance, Recognition, and Response • An animal must defend itself • From the many dangerous pathogens it may encounter in the environment • Two major kinds of defense have evolved that counter these threats • Innate immunity and acquired immunity

3. Figure 43.1 3m • Innate immunity • Is present before any exposure to pathogens and is effective from the time of birth • Involves nonspecific responses to pathogens

4. Acquired immunity, also called adaptive immunity • Develops only after exposure to inducing agents such as microbes, toxins, or other foreign substances • Involves a very specific response to pathogens

5. INNATE IMMUNITY Rapid responses to a broad range of microbes ACQUIRED IMMUNITY Slower responses to specific microbes External defenses Internal defenses Skin Phagocytic cells Humoral response (antibodies) Mucous membranes Antimicrobial proteins Secretions Inflammatory response Invading microbes (pathogens) Cell-mediated response (cytotoxic lymphocytes) Natural killer cells Figure 43.2 • A summary of innate and acquired immunity

6. Concept 43.1: Innate immunity provides broad defenses against infection • A pathogen that successfully breaks through an animal’s external defenses • Soon encounters several innate cellular and chemical mechanisms that impede its attack on the body

7. External Defenses • Intact skin and mucous membranes • Form physical barriers that bar the entry of microorganisms and viruses • Certain cells of the mucous membranes produce mucus • A viscous fluid that traps microbes and other particles

8. 10m Figure 43.3 • In the trachea, ciliated epithelial cells • Sweep mucus and any entrapped microbes upward, preventing the microbes from entering the lungs

9. Secretions of the skin and mucous membranes • Provide an environment that is often hostile to microbes • Secretions from the skin • Give the skin a pH between 3 and 5, which is acidic enough to prevent colonization of many microbes • Also include proteins such as lysozyme, an enzyme that digests the cell walls of many bacteria

10. Internal Cellular and Chemical Defenses • Internal cellular defenses • Depend mainly on phagocytosis • Phagocytes, types of white blood cells • Ingest invading microorganisms • Initiate the inflammatory response

11. Blood clot Pin Pathogen Macrophage Blood clotting elements Chemical signals Phagocytic cells Phagocytosis Capillary Red blood cell Fluid, antimicrobial proteins, and clotting elements move from the blood to the site. Clotting begins. 1 Chemical signals released by activated macrophages and mast cells at the injury site cause nearby capillaries to widen and become more permeable. Neutrophils and macrophages phagocytose pathogens and cell debris at the site, and the tissue heals. Chemokines released by various kinds of cells attract more phagocytic cells from the blood to the injury site. 4 2 3 Figure 43.6 Major events in the local inflammatory response

12. Pseudopodia surround microbes. 1 Microbes Microbes are engulfed into cell. 2 MACROPHAGE Vacuole containing microbes forms. 3 Vacuole Lysosome containing enzymes Vacuole and lysosome fuse. 4 Toxic compounds and lysosomal enzymes destroy microbes. 5 Microbial debris is released by exocytosis. 6 Phagocytic Cells • Phagocytes attach to their prey via surface receptors • And engulf them, forming a vacuole that fuses with a lysosome Figure 43.4

13. Concept 43.2: In acquired immunity, lymphocytes provide specific defenses against infection • Acquired immunity • Is the body’s second major kind of defense • Involves the activity of lymphocytes

14. Interstitial fluid bathing the tissues, along with the white blood cells in it, continually enters lymphatic capillaries. 1 Lymphatic capillary Interstitial fluid Fluid inside the lymphatic capillaries, called lymph, flows through lymphatic vessels throughout the body. 2 Adenoid Tonsil Lymphatic vessels return lymph to the blood via two large ducts that drain into veins near the shoulders. 4 Lymph nodes Blood capillary Spleen Lymphatic vessel Tissue cells Peyer’s patches (small intestine) Within lymph nodes, microbes and foreign particles present in the circulating lymph encounter macro- phages, dendritic cells, and lymphocytes, which carry out various defensive actions. 3 Appendix Lymphatic vessels Masses of lymphocytes and macrophages Lymph node Figure 43.5 The human lymphatic system

15. Antigen- binding sites Epitopes (antigenic determinants) Antibody A Antigen Antibody B Antibody C • An antigen is any foreign molecule • That is specifically recognized by lymphocytes and elicits a response from them • A lymphocyte actually recognizes and binds • To just a small, accessible portion of the antigen called an epitope Figure 43.7

16. Antigen Recognition by Lymphocytes • The vertebrate body is populated by two main types of lymphocytes • B lymphocytes (B cells) and T lymphocytes (T cells) • Which circulate through the blood • The plasma membranes of both B cells and T cells • Have about 100,000 antigen receptor that all recognize the same epitope

17. Clonal Selection of Lymphocytes • In a primary immune response • Binding of antigen to a mature lymphocyte induces the lymphocyte’s proliferation and differentiation, a process called clonal selection

18. Antigen molecules bind to the antigen receptors of only one of the three B cells shown. Antigen molecules B cells that differ in antigen specificity Antigen receptor The selected B cell proliferates, forming a clone of identical cells bearing receptors for the selecting antigen. Some proliferating cells develop into short-lived plasma cells that secrete antibodies specific for the antigen. Some proliferating cells develop into long-lived memory cells that can respond rapidly upon subsequent exposure to the same antigen. Antibody molecules Clone of memory cells Clone of plasma cells Figure 43.12 • Clonal selection of B cells • Generates a clone of short-lived activated effector cells and a clone of long-lived memory cells

19. 1 Secondary response to anti- gen A produces antibodies to A; primary response to anti- gen B produces antibodies to B Day 1: First exposure to antigen A 4 Primary response to antigen A produces anti- bodies to A 2 Day 28: Second exposure to antigen A; first exposure to antigen B 3 104 103 Antibody concentration (arbitrary units) 102 Antibodies to A Antibodies to B 101 100 35 28 21 42 49 56 0 14 7 Figure 43.13 Time (days) Primary and Secondary Immune Responses • In the secondary immune response, memory cells facilitate a faster, more efficient response

20. Humoral and cell-mediated immunity defend against different types of threats • Acquired immunity includes two branches • The humoral immune response involves the activation and clonal selection of B cells, resulting in the production of secreted antibodies • The cell-mediated immune response involves the activation and clonal selection of cytotoxic T cells

21. Cell-mediated immune response Humoral immune response First exposure to antigen Antigens displayedby infected cells Antigens engulfed and displayed by dendritic cells Intact antigens Activate Activate Activate Secreted cytokines activate B cell HelperT cell CytotoxicT cell Gives rise to Gives rise to Gives rise to Active and memory helperT cells Memory cytotoxicT cells Active cytotoxicT cells MemoryB cells Plasmacells Defend against infected cells, cancer cells, and transplanted tissues Secrete antibodies that defend againstpathogens and toxins in extracellular fluid Figure 43.14 • The roles of the major participants in the acquired immune response

22. Bone marrow Thymus Lymphoid stem cell T cell B cell Blood, lymph, and lymphoid tissues (lymph nodes, spleen, and others) Figure 43.10 Overview of lymphocyte development

23. Antibody-Mediated Disposal of Antigens • The binding of antibodies to antigens • Is also the basis of several antigen disposal mechanisms • Leads to elimination of microbes by phagocytosis and complement-mediated lysis

24. Binding of antibodies to antigens inactivates antigens by Viral neutralization (blocks binding to host) and opsonization (increases phagocytosis) Agglutination of antigen-bearing particles, such as microbes Activation of complement system and pore formation Precipitation of soluble antigens Complement proteins Bacteria Virus MAC Pore Soluble antigens Bacterium Foreign cell Leads to Enhances Cell lysis Phagocytosis Figure 43.19 Macrophage Antibody-mediated mechanisms of antigen disposal

25. Active and Passive Immunization • Active immunity • Develops naturally in response to an infection • Can also develop following immunization, also called vaccination

26. In immunization • A nonpathogenic form of a microbe or part of a microbe elicits an immune response to an immunological memory for that microbe

27. Passive immunity, which provides immediate, short-term protection • Is conferred naturally when IgG crosses the placenta from mother to fetus or when IgA passes from mother to infant in breast milk • Can be conferred artificially by injecting antibodies into a nonimmune person

28. Concept 43.4: The immune system’s ability to distinguish self from nonself limits tissue transplantation • The immune system • Can wage war against cells from other individuals • Transplanted tissues • Are usually destroyed by the recipient’s immune system

29. Table 43.1 • Recipient-donor combinations • Can be fatal or safe

30. IgE Allergen Histamine 1 3 2 Granule Mast cell Degranulation of the cell, triggered by cross-linking of adjacent IgE molecules, releases histamine and other chemicals, leading to allergy symptoms. 1 IgE antibodies produced in response to initial exposure to an allergen bind to receptors or mast cells. 2 3 On subsequent exposure to the same allergen, IgE molecules attached to a mast cell recog- nize and bind the allergen. Figure 43.20 Mast cells, IgE, and the allergic response

31. Autoimmune Diseases • In individuals with autoimmune diseases • The immune system loses tolerance for self and turns against certain molecules of the body

32. Figure 43.21 Rheumatoid arthritis • Is an autoimmune disease that leads to damage and painful inflammation of the cartilage and bone of joints

33. Other examples of autoimmune diseases: • Systemic lupus erythematosus (lupus) • Antibodies against a wide range of self molecules • Multiple sclerosis • Destruction of myelin sheath surrounding nerves • Insulin-dependent diabetes • Insulin- producing pancreas cells are targeted

34. Acquired Immunodeficiency Syndrome (AIDS) • Because AIDS arises from the loss of helper T cells • Both humoral and cell-mediated immune responses are impaired • People with AIDS • Are highly susceptible to opportunistic infections and cancers that take advantage of an immune system in collapse

35. 1µm Figure 43.22 • The loss of helper T cells • Results from infection by the human immunodeficiency virus (HIV)

36. The spread of HIV • Has become a worldwide problem • The best approach for slowing the spread of HIV • Is educating people about the practices that transmit the virus