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Defense Mechanisms 446

I. Nonspecific defense mechanisms include barriers to penetration of the body, as well as internal defenses.

A. Phagocytic cells engulf invading pathogens.

B. Interferons are polypeptides secreted by cells infected with viruses that help to protect other cells from viral infections.

II. Specific immune responses are directed against antigens.

A. Antigens are molecules or parts of molecules that are usually large, complex, and foreign.

B. A given molecule can have a number of antigenic determinant sites that stimulate the production of different antibodies.

III. Specific immunity is a function of lymphocytes.

A. B lymphocytes secrete antibodies and provide humoral immunity.

B. T lymphocytes provide cellmediated immunity.

C. The thymus and bone marrow are the primary lymphoid organs, which produce lymphocytes that seed the secondary lymphoid organs.

IV. Specific and nonspecific immune mechanisms cooperate in the development of local inflammation.

Functions of B Lymphocytes 453

I. There are five subclasses of antibodies, or immunoglobulins: IgG, IgA, IgM, IgD, and IgE.

A. These subclasses differ with respect to the polypeptides in the constant region of the heavy chains.

B. Each type of antibody has two variable regions that combine with specific antigens.

C. The combination of antibodies with antigens promotes phagocytosis.

II. Antigen-antibody complexes activate a system of proteins called the complement system. A. This results in complement fixation, in which complement proteins attach to a cell membrane and promote the destruction of the cell.

B. Free complement proteins promote opsonization and chemotaxis and stimulate the release of histamine from tissue mast cells.

Functions of T Lymphocytes 457

I. 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.

II. There are three subcategories of T lymphocytes.

A. 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.

B. 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.

C. Helper T lymphocytes stimulate, and suppressor T lymphocytes suppress, the function of B lymphocytes and killer T lymphocytes.

D. The T lymphocytes secrete a family of compounds called lymphokines that promote the action of lymphocytes and macrophages.

E. 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.

F. Histocompatibility antigens, or MHC molecules, are a family of molecules on the membranes of cells that are present in different combinations in different individuals.

III. 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 in combination with class-2 MHC antigens.

A. Helper T lymphocytes require such interaction with antigen-presenting cells in order to be activated by a foreign antigen; when activated in this way, the helper T cells secrete interleukin-2.

B. Interleukin-2 stimulates proliferation of killer T lymphocytes that are specific for the foreign antigen.

C. 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.

D. Interleukin-2 also stimulates proliferation of B lymphocytes, and thus promotes the secretion of antibodies in response to the foreign antigen.

Active and Passive Immunity 464

I. A primary response is produced when a person is first exposed to a pathogen. A subsequent exposure results in a secondary response.

A. During the secondary response, IgM antibodies are produced slowly and the person is likely to get sick.

B. During the secondary response, IgG antibodies are produced quickly and the person is able to resist the pathogen.

C. In active immunizations, the person is exposed to pathogens of attenuated virulence that have the same antigenecity as the virulent pathogen.

D. 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.

II. Tolerance to self-antigens occurs in prenatal development by destruction of T lymphocytes in the thymus that have specificity for the self-antigens.

A. This is called clonal deletion.

B. Clonal energy, or the suppression of lymphocytes, may also occur and may be responsible for tolerance to self-antigens by

B lymphocytes.

C. When tolerance mechanisms are ineffective, the immune system may attack self-antigens to cause autoimmune diseases.

III. Passive immunity is provided by the transfer of antibodies from an immune to a nonimmune organism.

A. Passive immunity occurs naturally in the transfer of antibodies from mother to fetus.

B. Injections of antiserum provide passive immunity to some pathogenic organisms and toxins.

IV. Monoclonal antibodies are made by hybridomas, which are formed artificially by the fusion of

B lymphocytes and multiple myeloma cells.

Tumor Immunology 468

I. Immunological surveillance against cancer is provided mainly by killer T lymphocytes and natural killer cells.

A. 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.

B. Natural killer cells are nonspecific, whereas T lymphocytes are directed against specific antigens on the cancer cell surface.

C. Immunological surveillance against cancer is weakened by stress.

Diseases Caused by the Immune System 470

I. Autoimmune diseases may be caused by the production of autoantibodies against self-antigens, or they may result from the development of autoreactive T lymphocytes.

II. Immune complex diseases are those caused by the inflammation that results when free antigens are bound to antibodies.

III. There are two types of allergic responses: immediate hypersensitivity and delayed hypersensitivity.

A. 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.

B. Mast cells secrete histamine, leukotrienes, and prostaglandins, which are believed to produce the symptoms of allergy.

C. Delayed hypersensitivity, as in contact dermatitis, is a cellmediated response of T lymphocytes.

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