Chapter Summary

Defense Mechanisms

1. Nonspecific defense mechanisms include barriers to penetration of the body, as well as internal defenses.
1. Phagocytic cells engulf invading pathogens.
2. Interferons are polypeptides secreted by cells infected with viruses that help protect other cells from viral infections.
2. Specific immune responses are directed against antigens.
1. Antigens are molecules or parts of molecules that are usually large, complex, and foreign.
2. A given molecule can have a number of antigenic determinant sites that stimulate the production of different antibodies.
3. Specific immunity is a function of lymphocytes.
1. B lymphocytes secrete antibodies and provide humoral immunity.
2. T lymphocytes proved cell-mediated immunity.

Functions of B Lymphocytes

1. There are five subclasses of antibodies, or immunoglobulins: IgG, IgA, IgM, IgD, and IgE.
1. These subclasses differ in regard to the polypeptides in the constant region of the heavy chains.
2. Each type of antibody has two variable regions that combine with specific antigens.
3. The combination of antibodies with antigens promotes phagocytosis.
2. Antigen-antibody complexes activate a system of proteins called the complement system.
1. This results in complement fixation, in which complement proteins attach to a cell membrane and promote the destruction of the cell.
2. Free complement proteins promote opsonization and chemotaxis and stimulate the release of histamine from tissue mast cells.
3. Specific and nonspecific immune mechanisms cooperate in the development of a local inflammation.

Active and Passive Immunity

1. A primary response is produced when a person is first exposed to a pathogen. A subsequent exposure results in a secondary response.
1. During the primary response IgM antibodies are produced slowly, and the person is likely to get sick.
2. During the secondary response IgG antibodies are produced quickly, and the person is able to resist the pathogen.
3. In active immunizations, the person is exposed to pathogens of attenuated virulence that have the same antigenicity as the virulent pathogen.
4. The secondary response is believed to be due to the development of lymphocyte clones as a result of the antigen-stimulated proliferation of appropriate lymphocytes.
2. Passive immunity is provided by the transfer of antibodies from an immune to a nonimmune organism.
1. Passive immunity occurs naturally in the transfer of antibodies from mother to fetus.
2. Injections of antiserum provide passive immunity to some pathogenic organisms and toxins.
3. Monoclonal antibodies are made by hybridomas, which are formed artificially by the fusion of B lymphocytes with multiple myeloma cells.

Functions of T lymphocytes

1. The thymus processes T lymphocytes and secretes hormones that are believed to be required for an effective immune response of T lymphocytes throughout the body.
2. There are three subcategories of T lymphocytes.
1. Killer T lymphocytes kill victim cells by a mechanism that does not involve antibodies but that does require close contact between the killer T cell and the victim cell.
2. Killer T lymphocytes are responsible for transplant rejection and for the immunological defense against fungal and viral infections, as well as for defense against some bacterial infections.
3. Helper T lymphocytes stimulate, and suppressor T lymphocytes suppress, the function of B lymphocytes and killer T lymphocytes.
4. The T lymphocytes secrete a family of compounds called lymphokines that promote the action of lymphocytes and macrophages.
5. Receptor proteins on the cell membrane of T lymphocytes must bind to a foreign antigen in combination with a histocompatibility antigen in order for the T cell to become activated.
6. Histocompatibility antigens, or MHC molecules, are a family of molecules on the membranes of cells that are present in different combinations in different individuals.
3. Antigen presenting cells, such as macrophages and dendritic cells, partially digest a foreign protein, such as a virus, and present the antigens to the lymphocytes on the surface of the macrophage in combination with class-2 MHC antigens.
1. Helper T lymphocytes require such interaction with macrophages in order to be activated by a foreign antigen; when activated in this way, the helper T cells secrete interleukin-2.
2. Interleukin-2 stimulates proliferation of killer T lymphocytes that are specific for the foreign antigen.
3. In order for the killer T lymphocytes to attack a victim cell, the victim cell must present the foreign antigen in combination with a class-1 MHC molecule.
4. Interleukin-2 stimulates proliferation of B lymphocytes, and thus promotes the secretion of antibodies in response to the foreign antigen.
4. Tolerance to self-antigens may be due to the destruction of lymphocytes that can recognize the self- antigens, or it may be due to suppression of the immune response by the action of specific suppressor T lymphocytes.

Tumor Immunology

1. Immunological surveillance against cancer is provided mainly by killer T lymphocytes and natural killer cells.
1. Cancerous cells dedifferentiate and may produce fetal antigens. These or other antigens may be presented to lymphocytes in association with abnormally produced class-2 MHC antigens.
2. Natural killer cells are nonspecific, whereas T lymphocytes are directed against specific antigens on the cancer cell surface.
3. Immunological surveillance against cancer is weakened by stress.

Diseases Caused by the Immune System

1. Autoimmune diseases may be caused by the production of autoantibodies against self-antigens, or they may result from the development of autoreactive T lymphocytes.
2. Immune complex diseases are those caused by the inflammation that results when free antigens are bound antibodies.
3. There are two types of allergic responses: immediate hypersensitivity and delayed hypersensitivity.
1. Immediate hypersensitivity results when an allergen provokes the production of antibodies in the IgE class. These antibodies attach to tissue mast cells and stimulate the release of chemicals from the mast cells.
2. Mast cells secrete histamine, leukotrienes, and prostaglandins, which are believed to produce the symptoms of allergy.
3. Delayed hypersensitivity, as in contact dermatitis, is a cell-mediated response of T lymphocytes.

After studying this chapter, students should be able to . . .

1. describe some of the mechanisms of nonspecific immunity and distinguish between nonspecific and specific immune defenses.
2. describe how B lymphocytes respond to antigens and define the terms memory cell and plasma cell.
3. describe the structure and classifications of antibodies and discuss the nature of antigens.
4. describe the complement system and explain how antigen-antibody reactions lead to the destruction of an invading pathogen.
5. describe the events that occur during a local inflammation.
6. describe the process of active immunity and explain how the clonal selection theory may account for this process.
7. describe the mechanisms of passive immunity and give natural and clinical examples of this form of immunization.
8. explain how monoclonal antibodies are produced and describe some of their clinical uses.
9. explain how T lymphocytes are classified and describe the function of the thymus.
10. define the term lymphokines and list some of these molecules and their functions.
11. describe the histocompatibility antigens and explain their importance in the function of the T cell receptor proteins.
12. describe the interaction between macrophages and helper T lymphocytes and explain how the helper T cells affect immunological defense by killer T cells and B cells.
13. explain the possible role of suppressor T lymphocytes in the negative feedback control of the immune response.
14. explain the possible mechanisms responsible for tolerance of self-antigens.
15. describe some of the characteristics of cancer and explain how natural killer cells and killer T lymphocytes provide immunological surveillance against cancer.
16. define the term autoimmune disease, give examples of different kinds of autoimmune diseases, and explain some of the mechanisms by which these diseases are produced.
17. explain how immune complex diseases may be produced and give examples of these diseases.
18. distinguish between immediate hypersensitivity and delayed hypersensitivity and describe the mechanisms responsible for each form of allergy.