CHAPTER 24 The Immune System Modules 24.1 – 24.2

The Continuing Problem of HIV Acquired immune deficiency syndrome (AIDS) is epidemic throughout much of the world 14,000 people are infected with the AIDS virus every day HIV is the virus that causes AIDS HIV is transmitted mainly in blood and semen Former L.A. Laker Magic Johnson is one of 900,000 Americans who are HIV-positive

Our immune system is a specific defense system It backs up several mechanisms of nonspecific resistance HIV attacks the immune system It eventually destroys the body’s ability to fight infection

NONSPECIFIC DEFENSES AGAINST INFECTION 24.1 Nonspecific defenses against infection include the skin and mucous membranes, phagocytic cells, and antimicrobial proteins The body’s first lines of defense against infection are nonspecific They do not distinguish one infectious microbe from another

Macrophages wander in the interstitial fluid They “eat” any bacteria and virus-infected cells they encounter Figure 24.1A

Interferon and complement proteins are activated by infected cells Viral nucleic acid VIRUS 6 Antiviral proteins block viral reproduction 1 2 Interferon genes turned on New viruses mRNA 3 5 Interferon stimulates cell to turn on genes for antiviral proteins Interferon molecules HOST CELL 1 Makes interferon; is killed by virus 4 HOST CELL 2 Protected against virus by interferon from cell 1 Figure 24.1B

24.2 The inflammatory response mobilizes nonspecific defense forces Tissue damage triggers the inflammatory response Skin surface Swelling Pin Phagocytes Bacteria Phagocytes and fluid move into area Chemical signals White blood cell 1 Tissue injury; release of chemical signals such as histamine 2 Dilation and increased leakiness of local blood vessels; migration of phagocytes to the area 3 Phagocytes (macrophages and neutrophils) consume bacteria and cell debris; tissue heals Figure 24.2

The inflammatory response can disinfect tissues limit further infection

The lymphatic system is a network of lymphatic vessels and organs 24.3 The lymphatic system becomes a crucial battleground during infection The lymphatic system is a network of lymphatic vessels and organs It returns tissue fluid to the circulatory system It fights infections

LYMPHATIC VESSEL Adenoid Tonsil VALVE Right lymphatic duct, entering vein Tissue cells Lymph nodes Interstitial fluid Blood capillary Thoracic duct, entering vein Thoracic duct Thymus Appendix LYMPHATIC CAPILLARY Spleen Masses of lymphocytes and macrophages Bone marrow Lymphatic vessels Figure 23.3

It will return it as lymph to the blood This lymphatic vessel is taking up fluid from tissue spaces in the skin It will return it as lymph to the blood Lymph contains less oxygen and fewer nutrients than interstitial fluid LYMPHATIC VESSEL VALVE Tissue cells Interstitial fluid Blood capillary LYMPHATIC CAPILLARY Figure 23.3B

Lymph nodes are key sites for fighting infection They are packed with lymphocytes and macrophages Masses of lymphocytes and macrophages Outer capsule of lymph node Macrophages Lymphocytes Figure 23.3C, D

24.4 The immune response counters specific invaders SPECIFIC IMMUNITY 24.4 The immune response counters specific invaders Our immune systems responds to foreign molecules called antigens Infection or vaccination triggers active immunity The immune system reacts to antigens and “remembers” an invader We can temporarily acquire passive immunity

24.5 Lymphocytes mount a dual defense Two kinds of lymphocytes carry out the immune response B cells secrete antibodies that attack antigens T cells attack cells infected with pathogens BONE MARROW Stem cell THYMUS Via blood Immature lymphocytes Antigen receptors T cell B cell CELL- MEDIATED IMMUNITY HUMORAL IMMUNITY Via blood Lymph nodes, spleen, and other lymphatic organs Final maturation of B and T cells in lymphatic organ OTHER PARTS OF THE LYMPHATIC SYSTEM Figure 24.5

24.6 Antigens have specific regions where antibodies bind to them Antigenic determinants are the molecules to which antibodies bind Antibody A molecules Antigen- binding sites Antigenic determinants Antigen Antibody B molecule Figure 24.6

24.7 Clonal selection musters defensive forces against specific antigens When an antigen enters the body, it activates only lymphocytes with complementary receptors B and T cells multiply into clones of specialized effector cells that defend against the triggering antigen This is called clonal selection

Antigen molecules Variety of B cells in a lymph node Antigen receptor (antibody on cell surface) Cell growth division, and differentiation Clone of many effector cells secreting antibodies Endoplasmic reticulum Antibody molecules Figure 24.7

24.8 The initial immune response results in a type of “memory” In the primary immune response, clonal selection produces memory cells These cells may confer lifelong immunity Figure 24.8A

When memory cells are activated by subsequent exposure to an antigen, they mount a more rapid and massive secondary immune response Unstimulated lymphocyte First exposure to antigen FIRST CLONE Memory cells Second exposure to antigen Effector cells SECOND CLONE More memory cells New effector cells Figure 24.8B

24.9 Overview: B cells are the main warriors of humoral immunity Triggered by a specific antigen, a B cell differentiates into an effector cell The effector cell is called a plasma cell The plasma cell secretes antibodies

PRIMARY RESPONSE (initial encounter with antigen) Antigen Antigen receptor on a B cell Antigen binding to a B cell Cell growth, division, and differentiation Clone of cells Memory B cell Plasma cell Antibody molecules Later exposure to same antigen SECONDARY RESPONSE (can be years later) Cell growth, division, and further differentiation Larger clone of cells Plasma cell Memory B cell Antibody molecules Figure 24.9

24.10 Antibodies are the weapons of humoral immunity An antibody molecule Figure 24.10A

An antibody molecule has antigen-binding sites specific to the antigenic determinants that elicited its secretion Antigen-binding sites Light chain Heavy chain Figure 24.10B

24.11 Antibodies mark antigens for elimination Antibodies may block harmful antigens on microbes clump bacteria or viruses together precipitate dissolved antigens activate complement proteins

Binding of antibodies to antigens inactivates antigens by Neutralization (blocks viral binding sites; coats bacterial toxins) Agglutination of microbes Precipitation of dissolved antigens Activation of complement Complement molecule Bacteria Virus Antigen molecules Bacterium Foreign cell Hole Enhances Leads to Phagocytosis Cell lysis Macrophage Figure 24.11

producing monoclonal antibodies 24.12 Connection: Monoclonal antibodies are powerful tools in the lab and clinic These molecules are produced by fusing B cells specific for a single antigenic determinant with easy-to-grow tumor cells Antigen injected into mouse Tumor cells grown in culture B cells (from spleen) Tumor cells Cells fused to generate hybrid cells Single hybrid cell grown in culture Antibody Hybrid cell culture, producing monoclonal antibodies Figure 24.12A

These cells are useful in medical diagnosis Example: home pregnancy tests They are also useful in the treatment of certain cancers Figure 24.12B

24.13 T cells mount the cell-mediated defense and aid humoral immunity Helper T cells and cytotoxic T cells are the main effectors of cell-mediated immunity Helper T cells also stimulate the humoral responses

Cell-mediated immunity An antigen-presenting cell (APC) first displays a foreign antigen and one of the body’s own self proteins to a helper T cell Microbe Macrophage (will become APC) 1 Antigen from microbe (nonself molecule) Self protein Self protein displaying antigen T cell receptor Binding site for self protein 3 2 Helper T cell 4 Binding site for antigen APC Figure 24.13A

The helper T cell’s receptors recognize the self-nonself complexes on the APC The interaction activates the helper T cells The helper T cell can then activate cytotoxic T cells with the same receptors Self protein displaying an antigen Cell-mediated immunity (attack on infected cells) Cytotoxic T cell T cell receptor Interleukin-2 stimulates cell division Interleukin-2 activates other T cells and B cells APC Helper T cell Humoral immunity (secretion of antibodies by plasma cells) B cell Interleukin-1 activates helper T cell Figure 24.13B

Cytotoxic T cells bind to infected body cells and destroy them 1 Cytotoxic T cell binds to infected cell 2 Perforin makes holes in infected cell’s membrane 3 Infected cell is destroyed Hole forming Foreign antigen INFECTED CELL Cytotoxic T cell Perforin molecule Figure 24.13C

24.14 Cytotoxic T cells may help prevent cancer Cytotoxic T cells may attack cancer cells The surface molecules of cancer cells are altered by the disease Figure 24.14

24.15 The immune system depends on our molecular fingerprints The immune system normally reacts only against nonself substances It generally rejects transplanted organs The cells of transplanted organs lack the recipient’s unique “fingerprint” of self proteins

DISORDERS OF THE IMMUNE SYSTEM 24.16 Connection: Malfunction or failure of the immune system causes disease Autoimmune diseases The system turns against the body’s own molecules Immunodeficiency diseases Immune components are lacking, and infections recur Physical and emotional stress may weaken the immune system

Allergies are abnormal sensitivities to allergens in the surroundings 24.17 Connection: Allergies are overreactions to certain environmental antigens Allergies are abnormal sensitivities to allergens in the surroundings B cell (plasma cell) Histamine Mast cell Antigenic determinant Antibodies attach to mast cell Allergen (pollen grain) B cells make antibodies Allergen binds to antibodies on mast cell Histamine is released, causing allergy symptoms SENSITIZATION: Initial exposure to allergen LATER EXPOSURE TO SAME ALLERGEN Figure 24.17

24.18 Connection: AIDS leaves the body defenseless The AIDS virus attacks helper T Cells This cripples both cell-mediated and humoral immunity So far, AIDS is incurable Drugs and vaccines offer hope for the future Practicing safer sex could save many lives