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Chapter 15

Chapter 15. The Immune System. 15-1. Active Immunity. 15-62. Primary and Secondary Responses. On 1st exposure to pathogen, there is latency of 5-10 days before specific antibodies are made (= primary response ) Antibody levels plateau after few days and decline after a few weeks

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Chapter 15

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  1. Chapter 15 The Immune System 15-1

  2. Active Immunity 15-62

  3. Primary and Secondary Responses • On 1st exposure to pathogen, there is latency of 5-10 days before specific antibodies are made (= primary response) • Antibody levels plateau after few days and decline after a few weeks • Subsequent exposure to same antigen causes secondary response • Antibody production is much more rapid and sustained 15-63

  4. Clonal Selection Theory • Is mechanism by which secondary immune responses are produced • Each B cell produces only 1 kind of antibody and related antigen receptor (on its surface) • Exposure to its antigen stimulates a B cell to divide until a large population of genetically identical cells (clones) is produced • Some of these become plasma cells and secrete antibodies • Some become memory cells that can be stimulated to produce antibodies in the secondary response 15-64

  5. 15-65

  6. 15-66

  7. Germinal Centers • Develop in lymph nodes and spleen from a cloned and activated B cell • Which proliferate and undergo hypermutation • Generating and secreting diverse antibodies for the 2o immune response 15-67

  8. Active Immunity • Development of a secondary response provides active immunity • Immunizations induce primary responses by inoculating people with pathogens whose virulence has been attenuated or destroyed (vaccinations) • Cause development of B cell clones that can provide 2o response 15-68

  9. Immunological Tolerance • Ability to produce antibodies against non-self antigens while tolerating self-antigens (immunological competence) occurs during 1st month of life • Tolerance requires continuous exposure to an antigen • Some self-antigens, such as lens protein in eye, are normally hidden from blood • Exposure to such self-antigens results in production of autoantibodies • Killer T cells that attack self-antigens are called autoreactive T cells 15-69

  10. Immunological Tolerance continued • 2 possible mechanisms for tolerance: • Clonal deletion theory: tolerance occurs because T cells that recognize self-antigens are destroyed • Good evidence this occurs in thymus • Clonal anergy: lymphocytes directed against self-antigens are present throughout life but don't attack self-antigens • Mechanism not understood • Appears to underlie tolerance in B cells 15-70

  11. Passive Immunity 15-71

  12. Passive Immunity • Is immune protection produced by transfer of antibodies to a recipient from a donor • Donor was actively immunized • Person who receives these ready-made antibodies is passively immunized • Used to treat snakebite, rabies, tetanus, hepatitis 15-72

  13. Passive Immunity continued • Occurs naturally before and after birth • Some antibodies from mother pass placenta to fetus during pregnancy and provide passive immunity • During 1st 2-3 days of nursing, mother produces colostrum which is rich in her antibodies and gives her immunity to infant • Immunological competence (ability to mount a specific immune response) does not develop until 1 month after birth 15-73

  14. 15-74

  15. Monoclonal Antibodies • Commercially prepared for use in research and diagnostic tests • Exhibit specificity for only 1 antigenic determinant • Animal (usually mouse) is injected with antigen, and its B cells harvested from spleen • Bs are hybridized with cancerous myeloma cells to make them immortal • Individual Bs are screened and the one that produces right antibody is selected • This one is allowed to multiply in culture and its clones (= hybridoma) are source of large quantity of antibodies 15-75

  16. 15-76

  17. Immune System and Cancer 15-77

  18. Tumor Immunology • Believed that tumor cells arise often but are normally recognized and killed by immune system • When cancer develops, the immunological surveillance system of T and natural killer cells has failed • Tumor biology is similar to and interrelated with functions of immune system • Most tumors are clones of single cells whose mitosis is not controlled by normal inhibitory mechanisms 15-78

  19. Tumor Immunology continued • Tumor cells dedifferentiate (become less specialized like cells of an embryo) • As dedifferentiate, produce surface antigens that are normally recognized by immunological surveillance and destroyed • Because were absent at the time immunological competence was established • Body treats these antigens as foreign • Presence of these antigens provides basis of laboratory diagnostic tests for some cancers 15-79

  20. Natural Killer (NK) Cells • Are lymphocytes related to T cells • Provide first line of cell-mediated defense • Considered to be part of the innate immune system • Possess an array of surface receptors that allow them to fight viruses, bacteria, parasites and malignant cells • NK cells destroy tumors in a non-specific fashion; backed up by specific response of killer Ts • NKs are stimulated by interferon from T cells • NKs attack cells that lack class-1 MHC antigens • Kill with perforins and granzymes 15-80

  21. Immunotherapy for Cancer • Most strategies involve boosting, or directing, patients own immune responses • Interferons and interleukins have been useful in treatment of some forms of cancer • E.g. interleukin-2, which activates both killer T and B cells, has shown promise 15-81

  22. Effects of Aging and Stress • Little is known about why susceptibility to cancer is so variable • Cancer risk increases with age • One factor may be that aging lymphocytes accumulate genetic errors that decrease effectiveness • Thymus function declines with age • Tumors grow faster in stressed animals • Stress hormones (corticosteroids) cause decreased immune function 15-82

  23. Diseases Caused By Immune System 15-83

  24. Diseases Caused by Immune System • Can be grouped into 3 categories: autoimmune diseases, immune complex diseases, and allergies • All caused by abnormal functioning of immune system 15-84

  25. Autoimmune Diseases • Are produced by failure of immune system to recognize and tolerate self-antigens • Autoreactive T cells are formed and B cells produce autoantibodies • Afflicts women twice as often as men 15-85

  26. Autoimmune Diseases continued • Failure of self-tolerance may be due to: • An antigen that does not normally circulate in blood being presented to immune system • E.g. in Hashimoto's thyroiditis, antibodies are stimulated to attack thyroglobulin (which is normally hidden from immune surveillance) 15-86

  27. Autoimmune Diseases continued • Failure of self-tolerance may be due to: • Combination of a self-antigen, that is otherwise tolerated, with a foreign hapten • E.g. in thrombocytopenia (low platelet count), platelets are destroyed because they combine with victim's medications 15-87

  28. Autoimmune Diseases continued • Failure of self-tolerance may be due to: • Antibodies being produced that are directed against other antibodies • Happens with rheumatoid arthritis 15-88

  29. Autoimmune Diseases continued • Failure of self-tolerance may be due to: • Antibodies against foreign antigens cross-reacting with self-antigens • This can happen with rheumatic fever 15-89

  30. Autoimmune Diseases continued • Failure of self-tolerance may be due to: • Self-antigens being presented to helper T cells together with class-2 MHC molecules • This happens in Type I diabetes 15-90

  31. Immune Complex Diseases • Involve formation of immune complexes that are free and not attached to a cell • These activate complement proteins and promote inflammation • Can result from infections by bacteria, parasites, viruses • Can result from formation of complexes between self-antigens and autoantibodies • This occurs in rheumatoid arthritis and lupus 15-91

  32. Allergy (Hypersensitivity) • Is an abnormal immune response to allergens • Comes in 2 forms: immediate and delayedhypersensitivity • Immediate is due to abnormal B cell response to allergen; causes effects in secs to mins • Caused by foods, bee stings, pollen • Delayed is abnormal T cell response that causes symptoms 24-72 hrs after exposure 15-92

  33. Immediate Hypersensitivity • Dendritic cells stimulate a class of helper Ts to secrete interleukin-4 and -13 which cause B and plasma cells to secrete IgE antibodies • IgEs do not circulate in blood; are attached to mast cells and basophils • When re-exposed to same allegen, antibodies on mast cells and basophils bind it and stimulate secretion of histamine, leukotrienes, and prostaglandin D • Producing allergy symptoms • Histamine increases capillary permeability and enhances immune response 15-93

  34. Immediate Hypersensitivity continued 15-94

  35. Delayed Hypersensitivity • Symptoms take longer to develop (hrs to days) • Is a cell-mediated T cell response • Symptoms caused by secretion of lymphokines, not histamine • Antihistamines provide little benefit • Examples include contact dermatitis caused by poison ivy, oak, or sumac 15-95

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