48.1 Many of the body's most effective defenses are nonspecific.
Skin: The First Line of Defense
• Skin, the largest organ of the vertebrate body, provides a nearly impenetrable barrier and reinforces defense with surface chemical weapons. (p. 1014)
• Mucous membranes of the digestive, respiratory, and urogenital tracts help trap and eliminate microorganisms before they invade body tissues. (p. 1014)
Cellular Counterattack: The Second Line of Defense
• Nonspecific cellular and chemical defenses respond to any microbial infection without the necessity of determining the specific identity of the invader. (p. 1015)
• Macrophages ingest microbes via phagocytosis; neutrophils ingest and kill bacteria by phagocytosis; and natural killer cells kill cells of the body infected with viruses. (p. 1015)
• The complement system consists of approximately 20 different blood proteins that aggregate to form a membrane attack complex, which then forms a pore in the foreign cell's plasma membrane. (p. 1016)
• Body cells infected by a virus secrete interferon to protect neighboring cells. (p. 1016)
The Inflammatory Response
• The inflammatory response is a localized, nonspecific response to infection. (p. 1017)
• Histamine and prostaglandins promote blood vessel dilation and capillary permeability, while a fever stimulates phagocytosis and causes the liver and spleen to store iron. (p. 1017)
48.2 Specific immune defenses require the recognition of antigens.
The Immune Response: The Third Line of Defense
• B cells (B lymphocytes) respond to antigens by producing antibodies that provide humoral immunity. (p. 1018)
• T cells (T lymphocytes) regulate the immune response of other cells, directly attack cells carrying specific antigens, and produce cell-mediated immunity. (p. 1018)
• Active immunity can be gained due to exposure to a pathogen, while passive immunity occurs when one individual obtains antibodies from another individual. (p. 1018)
Cells of the Specific Immune System
• T cells and B cells are both produced in bone marrow, but B cells develop and mature in bone marrow, while T cells complete development in the thymus. (p. 1019)
Initiating the Immune Response
• Helper T cells are activated when antigen-presenting cells present foreign antigens with MHC-II proteins. (p. 1020)
• Cytotoxic T cells interact with foreign antigens presented with MHC-I proteins. (p. 1020)
48.3 T cells organize attacks against invading microbes.
T Cells: The Cell-Mediated Immune Response
• In the cell-mediated immune response, cytotoxic T cells kill abnormal or virus-infected body cells. (p. 1021)
• Interleukin-1 is secreted by macrophages and stimulates helper T cells, while interleukin-2 is secreted by helper T cells and activates cytotoxic T cells and B cells. (pp. 1021-1022)
48.4 B cells label specific cells for destruction.
B Cells: The Humoral Immune Response
• B cells place markers on pathogens to alert complement proteins, macrophages, and natural killer cells to attack those cells. (p. 1023)
• When B cells recognize antigens, they produce plasma cells and large numbers of circulating antibodies to fight the antigens. (p. 1023)
• Antibodies are proteins in a class known as immunoglobulins, which is further divided into subclasses. (p. 1023)
• IgM antibodies are produced first and activate the complement system, while IgG antibodies are produced later to promote phagocytosis. (p. 1023-1024)
Antibodies
• Each antibody molecule consists of light chains and heavy chains containing variable and constant regions. Most of the sequence variation between antibodies is found in a cleft that only allows a specific antigen to bind. (p. 1025)
• Somatic rearrangement and DNA mutations assemble the genes that encode for the huge numbers of antigen receptors. (pp. 1025-1026)
• Immunological tolerance occurs when an animal's immune system accepts the animal's own tissue; it can occur through clonal deletion or clonal suppression. (p. 1026)
• A primary immune response occurs the first time the body encounters a pathogen and the binding of an antigen stimulates cell division (clonal selection). As a result of the first infection, the body is prepared to recognize and fight off the same pathogen the next time it is encountered (secondary immune response). (p. 1026-1027)
Antibodies in Medical Diagnosis
• Blood type denotes the class of antibodies found on the surface of red blood cells. The major group is known as the ABO system, while another system is the Rh factor. Mixing different blood types can lead to agglutination. (p. 1029)
• Monoclonal antibodies exhibit specificity for one antigen and are commercially prepared for medical diagnosis and research. (p. 1030)
Evolution of the Immune System
• Early organisms utilized a cellular immune response. (p. 1031)
• Vertebrates appear to be the first group to evolve an immune system based on lymphocytes. (p. 1032)
48.5 The immune system can be defeated.
T Cell Destruction: AIDS
• HIV inactivates CD4+ T cells and thus leaves the immune system incapable of mounting a response to a foreign antigen. (p. 1033)
• HIV exhibits a high mutation rate resulting in antigen shifting, thus making it difficult to develop an effective vaccine. (p. 1034)
Antigen Shifting
• A pathogen may escape recognition by the immune system if it changes its surface antigens. Such antigen shifting is an example of evolution by natural selection. (p. 1035)
Autoimmunity and Allergy
• Autoimmune diseases are produced by a failure of the immune system to recognize and tolerate self-antigens, and can result from a variety of mechanisms. (p. 1036)
• Allergies, or hypersensitivity, can be divided into immediate hypersensitivity and delayed hypersensitivity, both of which can cause the release of histamine. (p. 1036)
• In extreme cases, the widespread release of histamine can lead to anaphylactic shock. (p. 1036)